The top falls. At the very close of the movie, Cobb (Leonardo DiCaprio) returns to the U.S. and is reunited with his estranged children. Before greeting them, he spins his totem, a top that falls down whenever he’s in reality and keeps spinning when he’s in a dream. The movie cuts away before we can see what happens. If you follow this particular “Inception" theory, then the top ended up dropping. Cobb was not in a dream, which means that he successfully carried out his mission, gained entrance back to the United States, and now can move forward with life back in his home country.
The top kept spinning. If Cobb’s totem never fully dropped, then he still remains in the dream world. It is possible that after the inception mission was successfully carried out, that Cobb chose to remain in a dream world that allowed him to reunite with his children. He might even have still been trapped in limbo. Evidence in support of this “Inception” theory is the fact that his children never aged or changed their clothes throughout the movie. Cobb might have just kept this image of them and projected it into his dream world at the end.
It was all a dream. The world of “Inception” is never very clearly defined. Many scenes move right into one another without much explanation, which is what Cobb himself described as part of a dream’s consistency. The entire movie could have been Cobb’s own dream, according to this “Inception” theory. Under this theory, Mal (Marion Cotillard) might not have gone crazy at all with her belief that the couple was trapped in a dream. Instead of killing herself, she could have returned to the real world, while Cobb stayed behind in the dream.
It doesn’t matter. The movie cuts to credits right before we see what happened to the top in order meant to provoke questions, no doubt. Director Christopher Nolan might have used this ending to deliver another message that the top's outcome doesn’t truly matter. Cobb had always been obsessed with testing himself to ascertain if he was in reality or a dream. It is surprising that this time, he simply walks away before getting an answer. Not only does the audience remain unsure if it’s a dream, but Cobb doesn’t truly know if he’s in a dream either. He is finally at peace with himself and reunited with his children, and this is all the reality that he needs.