Meetings are gatherings of members of a clan that is organized by a Major or a Commander. The purpose of a meeting is usually to inform the members of something that has happened recently, or just go give a general update on how the clan is doing. Once you're a fully-fledged member of the community (Corporal +) you are expected to attend the meetings whenever they're called.
If you cannot attend a meeting that's fine, but please make sure you notify whoever is hosting it.
Squad Meetings happen only when the Majors want to announce something.
Clan Meetings happen every week.