The movie focuses on Cobb's big final heist, wherein he and his operatives must reach the deepest level of a tycoon's subconscious and implant an idea. This is actually the least complicated part of the film.
Throughout the movie, the big problem lurking below the surface is that Cobb is haunted by the memories of his dead wife, Mal, which threatens to screw up the whole operation. To make sure this doesn't happen, much of the final plan is hidden from Cobb himself. So Ellen Page's character, Adriadne, knows a way into the mind fortress of the tycoon, but Cobb doesn't. It's for their own protection -- if Cobb knows, his memories of Mal will fuck everything up.
Phantom dream women, right?
Once Cobb and his Dream Bandits make it inside the tycoon's subconscious, they get intercepted by a bunch of dudes on snowmobiles trying to keep them out of the mountain bunker where the tycoon's innermost secrets are stored (because without question, this is how everyone's mind is organized). With everything going to hell, Cobb demands that Adriadne tell him the one thing he can't be allowed to know: the way into the complex. This exchange happens:
ARIADNE: I don't think I should tell you. If Mal finds out ...
COBB: We don't have time for this! Did he add anything?
ARIADNE: He added an air duct system that can cut through the maze.
COBB: Good. [points to her radio] Explain it to them.
"Don't explain anything to the audience, though. They like being kept in the dark."