I think that this article from the SE blog explains them well

[The Death of Meta Tags][1]

> 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 [tag:devops] which without some other tag would be meaningless on an SF question.


  [1]: http://blog.stackoverflow.com/2010/08/the-death-of-meta-tags/