That's a comment, using the meaning of the word from the context of programming. I can't find anything about that in the editing help page, but it's a standard HTML technique which is supported in Markdown. <!--stuff--> is a simple comment; if I remove the backticks from the markdown to get rid of the code formatting, it won't show up in the content.
Some anonymous user wanted to make a one-character edit, which is forbidden by the 6-character limit, and putting comments like that are a standard way of 'cheating'.
Ideally, edits which do not satisfy the 6-character limit through visible, useful changes should neither be made nor approved, since the limit is implemented to avoid such trivial edits in the first place. If you find yourself noticing such a typo like that, you could either leave a comment (comments are to suggest improvements!), or find other parts of the post which could be improved. But frivolous HTML tags are certainly a bad idea. If you have >2k rep, you can technically do it, but you should avoid it (the 6 character limit is not implemented for those with the editing privilege), because your edits will still bump the post to the top of the front page.
Now that it's there, it's an ugly blot in the markdown, but I still don't feel it's worth bumping the post to the front page by just removing that comment. If you, or any other future editor has a more legitimate thing to change, it would be a perfect opportunity to delete that bit.
