How do comments work?
I mean, comment on a comment? How do I do that?
You need 50 rep to comment globally, unless it's either …
an answer to your question
… in which case you can comment with 1 rep.
Meta has a different ruleset from the other trilogy sites and allows global commenting with 1 rep. All rep levels are documented on the /faq page as usual.
(shamelessly stolen from mmeyers' comment, but he should have left it as an answer!)
You have to have 50 reputation before you can leave a comment on stackoverflow, but you only need 1 on meta.
I understand that you mean : To comment not on a question or answer, but on one of theirs comments.
What I've seen, and am doing also, is to prefix my answer with
author is the name of the person who wrote the comment.