The artist comment is a field in the database used to help users distinguish between identically named artists.
You will be prompted to enter a comment any time you enter a new artist which shares a name with an existing artist.
The artist comment will appear in the search results next to each name: so, ideally, the comment should be kept fairly short. A few words is normally enough, just enough that someone reading it will recognize the artist they're looking for. The comment field is not a place to store general background information about the artist: that kind of information should go in the ArtistAnnotation.
Note that the system will automatically prompt you to enter a comment for the new artist, but not for the old one. To make the search results a little clearer, it's a good idea to go back and enter a comment for the old artist as well. This can be done on the EditArtistPage of the old artist.
In order to distinguish between Randy Jackson, brother of Michael Jackson and Janet Jackson, and Randy Jackson, former bassist with Journey, we use artist comments. The former has comment "Brother of Michael and Janet." while the latter has "Former bassist with Journey".
Note that some concerns have been voiced about the choice of the name ArtistComment.