I have heard this term bandied around, but I have not seen a good definition.
What are "meta tags"?
Meta tags are tags that describe the question itself rather than the topic of the question. They're so named in the same way our Meta site is named — here we have questions about the site rather than questions within the site's topic.
Imagine we had the following question:
What's the best custom ROM for the Galaxy S?
On the other hand, a tag like subjective-question would be a Meta tag because it describes only the question itself, despite accurately doing so.
An example from Stack Overflow is homework. All that should matter is the development issue that's being asked about, not the reason the development is being done (school, work, hobby, whatever). Meta tags distract from solving the problem.