What is a .md file? A plain-English explanation

A .md file is an ordinary text document written in a style called Markdown. The symbols you see in it, such as #, **, - and |, are instructions for how the text should look: this line is a heading, this word is bold, these lines are a list. An app that understands Markdown hides the symbols and shows the formatting. An app that does not, such as TextEdit, shows you the symbols themselves.

Markdown in one paragraph

Most documents you know, such as Word files and PDFs, keep their formatting in a hidden layer you never see. Markdown does the opposite: the formatting is written into the text itself, using ordinary keyboard characters. The file is just text, so any computer can open it, it takes up almost no space, and it will still be readable in thirty years. The price is that you need an app that reads the symbols and turns them into formatting, or you see them raw.

People who build software call this kind of thing a “markup language”: a set of marks you add to text to describe its layout. HTML, the language behind web pages, is another one, and a much fussier one. The name Markdown is a play on the word.

Who invented it, and why

Markdown was created by John Gruber, the writer behind the blog Daring Fireball, and released in 2004, with a great deal of help from the programmer Aaron Swartz. Gruber wanted a way to write for the web without typing HTML, and he set one rule above the others: a Markdown document should be readable as it is, as plain text, without looking as if it has been covered in formatting instructions.

So the symbols were borrowed from habits people already had in plain-text email, such as asterisks around a word for emphasis. That is why a raw .md file, for all its clutter, is still mostly readable.

Other people later added features the original lacked. The version most widely used today is GitHub Flavored Markdown, which added tables, checklists and strikethrough. When people say “Markdown” now, they usually mean that.

Why AI tools write in Markdown

When ChatGPT, Claude, Gemini or Copilot answers you, the text it writes is usually Markdown. You rarely notice, because the chat window turns the symbols into headings, bold text and tables before you see them. It suits them well: it is compact, it survives being copied between apps, and it is common in the kind of technical writing these tools learned from.

The trouble starts when the answer leaves the chat. Ask for “a file” and you get a .md: the same text, symbols included, with nothing to convert them. Copy an answer into an app that does not understand Markdown and the asterisks come along too. If that is what brought you here, why AI answers are full of asterisks covers the copy-and-paste side in more detail.

What the symbols mean

These are the ones you will meet in almost every AI-written document.

You see in the file	It means
`# Title`	The main heading
`## Section`, `### Subsection`	Smaller headings: the more `#`, the smaller the heading
`important`	Bold: important
`note` or `_note_`	Italics: note
`- item` or `* item`	A bulleted list
`1. item`	A numbered list
`- [ ] task` and `- [x] done`	A checklist: an empty box, and a ticked one
`[the report](https://example.com)`	A link: the words go in square brackets, the address in round ones
`> quoted text`	A quote, shown indented
`code`	A technical term or a bit of code, shown in a typewriter-style font
Three backticks on a line of their own	The start or end of a block of code, shown in a shaded box
Lines of words separated by vertical bars	A table (see below)

Tables look the most alarming in raw form, so here is one:

| Item   | Price |
| ------ | ----- |
| Coffee | £3    |
| Cake   | £4    |

The first line is the header row, the line of dashes separates it from the rest, and each line after that is a row. The bars mark where one column ends and the next begins. Displayed properly, it is a small, ordinary table with two columns and two rows.

A few others turn up now and then: ~~text~~ for strikethrough, --- on a line of its own for a dividing line, and ![description](picture.png) for an image. That covers nearly everything.

Why your Mac shows the symbols

macOS has no Markdown reader built in. As far as the system is concerned, a .md file is plain text, so double-clicking it opens whatever app handles plain text, usually TextEdit, or Xcode if Apple’s developer tools are installed. Both show you exactly what is in the file, symbols and all. Pressing Space in Finder does the same, unless an app has added a Markdown preview. The file is fine; the Mac simply does not know how to display it.

How to open one properly

Once, without installing anything: open it in TextEdit and read around the symbols. Tolerable for a short note, miserable for a long table.
In the cloud: Google Docs can open a .md file once you upload it to Google Drive, and since 2026 OneDrive and SharePoint can show and edit Markdown in the browser. Both work, and both mean uploading the file somewhere, which for a private document is a real decision.
In a code editor: VS Code shows a formatted preview if you press Shift+Cmd+V. It works, but it is a programmer’s tool and looks like one.
In a Markdown app: Hem is a free, open source Mac app made for this. On first launch it offers to become the app that opens .md files; after that, a double-click shows a formatted page, and pressing Space in Finder shows a formatted preview too. The file stays on your Mac. Hem is Mac only, so on Windows or a phone you will need something else.

A .md file open in Hem: the symbols are gone, and the report shows a heading, a table, a list and numbered steps.

How to open .md files on a Mac compares these options in more depth.

How to edit one

Any text editor can change a .md file, because it is only text. The catch is that you are editing the symbols: to make a word bold you type two asterisks on either side of it, and you need a preview to check the result.

If you would rather not learn any of that, use an app that hides the symbols while you work. In Hem you click into the formatted page and type, much as you would in Pages or Word. Selecting text brings up a small bar with bold, italic, headings, lists, checklists, links and quotes; Cmd+B and Cmd+I work as you would expect; Format > Insert Table adds a table; checklist boxes tick with a click. Hem saves as you go, into the same .md file, and rewrites only the parts you changed. The file stays ordinary Markdown that an AI tool or any other app can still read.

How to convert one

To Word: in Hem, File > Export as Word Document saves a .docx. Converting .md to Word on a Mac covers the other routes, including Google Docs.
To PDF: File > Export as PDF in Hem.
Into an email or another document: copy from Hem with Cmd+C and paste into Mail, Word, Pages or Google Docs. Headings, bold, lists and tables arrive as formatting, not symbols.
Back to an AI: there is no need to convert it. AI chat tools read Markdown comfortably, so paste or attach the .md file as it is.

The short version

A .md file is a text document with its formatting written in as symbols. It is not broken, not dangerous, and not only for programmers. It just needs an app that reads the symbols, and on a Mac you have to add one, because none comes built in.