Meta in this context is taken to mean referring to itself or to the conventions of its genre; self-referential.
It can be subtle. Duncan Jones' film Moon plays on the viewers potential mistrust of the moon base's AI - informed by the knowledge that the viewer has seen HAL in 2001. As Christopher Nolan said of Interstellar "it's impossible to make a movie on these themes without having a conservation with 2001"
Cronenburgh's version of The Naked Lunch is meta - it's not a direct adaptation, but partly tells the story of the book's writing.
Strangely enough, one of the few brilliant bits of Scott's Prometheus (amongst much frustrating stupidity) is when Micheal Fassbinder's android David watching David Lean's Lawrence of Arabia. It's meta (Ridley Scott nodding to an undisputed masterpiece of cinematography, itself an adaptation of a very self aware and deliberately idiosyncratic book that was rewritten many times), but carries hints at the plot ahead (an android watching a cinematic depiction of a man who in his own words felt angst over 'serving two masters'.)
The other brilliant part of Prometheus is of course Idris Elba playing a concertina. The rest was a mess.