What exactly is Michael Keaton's Birdman about?

I saw Birdman last night, as part of a renewed focus in December on seeing movies that will likely get nominated for a slate of Oscars. It was good, and interesting, and you can argue that a handful of people in the movie deserve to be nominated for some awards -- Michael Keaton, yes, but also Ed Norton and Emma Stone and Zach Galifianakis. (Not all of …