Meta tags are tags which describe something other than the question's content -- such as the author's skill level, the type of a post (e.g. big-lists, self-study), etc. Alternatively, they may be ill-defined concepts which make consistent application of the tag difficult or impossible.
See The Death of Meta Tags for the official party line on meta tags, and the broader category of "dependent" tags.
Reference: excerpt and wiki adapted from the main Meta Stack Exchange site.