If we're to understand what the ending of Inception really means we first need to go back to the beginning because the first act of this movie establishes three key aspects of Cobbs character. So, first off right from the opening shot the first thing we see is Cobb washed up on the shores of limbo and the first thing he sees is his children so we understand that his relationship with his children will be an important part of the story and later on Cobb gets a call from his children that confirms what we already suspected he is for whatever reason estranged from his children.

The second important aspect of Cobbs character that's presented and the thing that is in my opinion the most important aspect of his character is his relationship with reality. As soon as the opening action sequence is over we get to see Cobb alone and the first thing he does is spin the spinning top to confirm that he's not in a dream so here the story is planting the seed, the seed of the idea that Cobb doubts his reality he's never completely sure about whether or not he's in a dream which is we learn later on an idea that Cobb implanted in molls mind leading to her suicide which has haunted him ever since and this point is made again later on after Cobb awakens from the dream he's in with Ariadne and when he awakens from testing use of stroke as well he immediately goes to check the totem to make sure he's not in a dream.