So that’s why it’s so complicated. Cobb’s team in the movie needs to synchronize a pair of kicks for every level of the dream and for every person. They achieve the synchronizing by playing music in the ears of the dreamer. The music gives the cue to the team in the level below to perform a kick to synchronize with the level above.
The synchronized kicks are not required because of the multiple levels of dreams, it’s simply because the sedative is strong.
Death
Now the next catch is death. What happens if you die a the dream? Again this differs based on the type of the dream sequence.
If you die a basic dream, it simply works as a kick (to the dreaming self) and you wake up by one level (eg: Arthur is shot in the head).
However, if you die in a multi-level dream (strong sedative), you end up in Limbo.
Limbo
Limbo is a subconscious state where the mind might fail to tell that it’s a dream and can be lost there forever.
Now what happens if you die in Limbo? If death happens in limbo then you wake up all the way into reality bypassing all the levels of dreams.
Limbo is where Cobb and Mal have once been lost for almost a lifetime (50 years). The buildings you see there are what the two of them had built. Limbo is not one person's dream in particular. Its the common place that you end up in if you die in a heavily sedated dream.
Again there are two ways to get into Limbo. One way - If the architecture of the multi level dream consists of 3 levels and if you try to go one level deeper that what was designed, you can reach limbo. In this case you will still remember how you got to Limbo and what you were doing before that (eg: when Cobb and Ariadne go from the hospital level further down to get Fischer). Other way - If you die in a heavily sedated, multi-level dream you will be thrown into limbo but in this case you will not remember clearly how you got there and what you were doing before that (eg: when Saito dies and reaches limbo, he ends up living there for many years).
You cannot reach limbo in a basic dream. You simply cannot.
Inception
Now getting to the whole point of the movie. This is a method suggested by Cobb in the movie. He says that at every level of dream there is a safe house, which gets populated with the inner most thoughts and secrets of the subject. Cobb’s team usually extracts secrets from the subjects mind from the safe. Cobb also suggests that if you insert an idea into this safe at the 3rd level in the dream or below, the subject will actually wake up believing the idea is his or hers.
When Cobb and Mal end up in limbo, Cobb eventually realizes that they are not in reality. But Mal refuses to believe that. So Cobb locates Mal’s safe and leaves a perpetually spinning top there (perpetually spinning top is Mal's totem that indicates that Mal is in a dream). So in the limbo, Mal gets convinced that she is in a dream and decides to die with Cobb on the rail tracks and they both wake up in their room (reality). What Cobb does unknowingly is, the idea he incepts in Mal’s mind (safe), is now with her even after she has woken up. So Mal continues to think the real world is a dream and decides to die so she can wake up. This is how Cobb realizes that inception is possible.
Why is Saito so much older than Cobb in Limbo?
Saito gets shot in the 1st level and is dying. There is a misconception that Cobb dies in Level 4 (Limbo) as a result of Mal’s stabbing. That is not the case. When everyone rides their kicks back up to the 1st level, they exit the van with oxygen cans. They leave Cobb there to drown. There is a specific moment that captures a drowning Cobb, left to die. The cause of death for both Saito and Cobb is in the 1st level, the van level. For Saito it’s the bullet, for Cobb it’s the drowning. A short moment of time lapses between both of their deaths. But in Limbo, due to time dilation, this is in years. This is why Saito is so much older in Limbo as compared to Cobb.
Ending Explained – The Spinning Top
Look at it this way. If it was all a dream at the end, then we don't know when the dream started, whose dream it was and if there were any other “real people” or merely projections, and if projections then whose projections? If the whole movie were a dream, the brilliant concept behind the levels of dreams and kicks would all go waste. So, given that we are shown a spinning top with an unclear mention of whether Cobb is still in a dream or not, it would simply make the movie a better one to choose that the entire movie was not a dream.
Besides, the top wobbles - that happens when it is slowing down. You would not have a wobble on a spinning top if its speed remained constant. Remember how it looked like in Mal’s safe? The top wobbles which does mean it’s eventually going to stop.
Story Source;http://www.thisisbarry.com/single-post/2015/09/15/Inception-2010-Explained