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!)