To comment is to state your opinion or make a remark on something. Of course, it's good to know when to hold your tongue. Comment on the latest movie? No problem. Comment on the annoying airport security measures while having your baggage checked? Better not.
A written explanation or criticism is a comment. Your teacher's comments on your paper may thrill you when you do excellent work or unnerve you when you receive the message, "See me!" If you state your opinion, this is also a comment. Politicians talking to the press are fond of the statement, "No comment," which means they don't want to express an opinion — or more likely, they don't want to get in trouble.