What is the "Wake Up" at the end supposed to mean?

The whole movie is a dream of one of them, but it's up to your decision whose it is. (at least I think this way)

Essentially "Wake Up" means that someone dreamed the whole thing.

I believe they intended this as a joke (quite successful in my opinion) on the expense of those movies, which resolve their incredibly complicated and twisted stories, by pronouncing that they were just a dream or an illusion.