The disambiguation comments are fields in the database used to help distinguish identically named artists, labels and other entities. They are visible in the pages for the entities, and also appear in the search results next to their names.
Disambiguation comments should be kept fairly short. A few words are usually enough for someone reading it to recognize the entity they're looking for. The comment fields should not be used to store general background information: that kind of information should go in the annotation instead. The text inside the comment field is not part of the title, and thus should be entered following the normal capitalization rules (that is, usually in lowercase).
You will be prompted to enter a disambiguation comment any time you try to add a new artist or label that shares its name with an existing one, but you'll not be prompted to enter a disambiguation comment for the "old" one, nor for works, recordings, releases and release groups. To enhance the search results, you should ideally provide a disambiguation comment for all the identically named entities, by navigating to their pages and editing them.