Comparing two documents
Comparison puts two of your open documents side by side and marks what differs between them: lines added on one side, lines removed on the other, with a glyph in the margin as well as the highlight, so the difference is readable either way. It works on the markdown itself rather than on the formatted preview, so what you are shown is exact. Documents get here from the editor: open or create the two you want to compare, then choose Compare documents. Both pickers list everything you currently have open, and you can switch either side without going back. If fewer than two documents are open, this screen says so and sends you back to the editor to open another. Comparison happens in your browser, like everything else here — neither document is uploaded anywhere.
Markdown files usually end in .md, which is why people call them md files. Comparing two md files here needs nothing installed: the page runs in a browser tab, like the editor it belongs to.