It seems to be a shortcut to open the comment url. Specifically opens the full article but at the height of the comment from which the link was generated, by pressing the icon.
There is a bug and the links are not being generated correctly, because they lack the part of the url that refers to the publication in which the comment has been made.
I will now make a comment to have a practical example.
If I try to open this comment in a new browser tab by pressing the Fediverse icon above, it redirects to https://lemmy.ml/comment/166757 that doesn't exist.
Edit: After reading some answers I have understood that the link does exist, but not to be opened directly from a browser, but to access the answer from other Fediverse platforms. What doesn't make sense is that if you press It from the Lemmy web interface it leads to a 404 error.
The correct address is https://lemmy.ml/post/241826/comment/166757 the same that is generated and copied to clipboard by pressing the icon in the form of links near the Fediverse icon.