‘Meta’ isn’t an abbreviation. It’s short for ‘metafandom.’ Way back in the dawn ages before Tumblr was invented, ‘metafandom’ was the phrase used for when fandom was analyzing the behavior of fandom.
Eventually the word ‘meta’ got associated with the analysis part, rather than the ‘fandom studies’ part. So it has become a piece of slang that can apply to any sort of fandom-based analysis.
So basically, it means literary analysis.