Meta tags are tags which describe something other than the question's content--such as the author's skill level, motivation, or current take.

A good test of a meta tag is remove all other tags from the question - does the tag make sense to describe what is truly being asked. Would another tag perhaps be better to atteract someone to assist in troubleshooting the actual tag in question?

A good example of previous meta tags on this site are:

But what is the beginner trying to do?
But what is the homework on?
Which tag covers the items undergoing troubleshooting?