Title says it all; I've seen lots of folks on meta SF and SO talking about these tags and how bad they are, but a formal definition has eluded me. The obvious definition (based on the meaning of the "meta" prefix) would be "tags of tags," but that doesn't seem to be what's going on here.
I think that this article from the SE blog explains them well
Meta-tags are actually a subset of a larger problem that I usually call dependent tags. These are tags that don’t say anything by themselves – you can’t tell what the question is about unless they’re paired with some other tag (or several of them). These tags are a problem because people don’t realize this and will often use that as the question’s only tag.
A recent example would be devops which without some other tag would be meaningless on an SF question.