Don't read if you haven't seen it yet!
Saito dies at some level (if I remember right at the top dream level) and he enters limbo. Cobb goes there from the snow compound dream level but Saito has already been there 20-40 years (maybe they said in the movie, I forgot that too). Cobb then helps him remember Saito is in a dream so they can wake up together from the limbo state.