Because their theft will require them to be within the dream world for a prolonged period of time as well as their plans to go multiple levels deep within the dream, they are required to be sedated.   In normal dream sharing, they always have the safety valve of dying, where if they die they are forced to wake up from the dream.  The problem for them on this endeavor comes in the fact that if they die while in this prolonged dream while sedated, they won’t wake up, but instead will fall into some limbo world.  That alone wouldn’t be so bad but for the fact that time travels much slower in dreams than in the real world.  In their first level of dreaming, time moves at the rate of 1 day in the dream for each hour in the real world.  Their plan is to go up to three levels deep in the dream, with each progressive level experiencing a more extensive time dilation effect.  In the worst case scenario, limbo,  entire lifetimes would pass by before they woke up.  Their minds would literally be lost and be stuck there so long they could no longer distinguish reality from dream.  With that in mind, each person carries a personal totem that they alone know the shape and feel of in order for them to distinguish reality from dream.  Cobb’s totem is a top that when spun in the dream will never fall over.

Cobb’s prior experience with Inception is also tied to his subconscious demons that follow him into each job.  In his past, he shared dreams with his wife.  They spent entire lifetimes inside their dreams together.  In the end, Cobb had to convince his wife that the dream world wasn’t real and they needed to return to reality.  Once returned, however, Cobb found that his wife retained that thought, planted there by him, that the world was not real.  In other words, Cobb had successfully achieved Inception.  The tragic result of his Inception though was his wife’s rejection of reality and subsequent suicide.  She had planned on convincing Cobb to join her in the suicide by attempting to blackmail him into it.  Cobb rejected the attempt and instead has spent his life since then on the run from the law who blames her death on him.

In the end of their theft adventure, Cobb voluntarily heads into limbo in order to look for Saito, who unfortunately died in the dream world too early and is lost there.  Cobb finds Saito, now grown extremely old.  He is able to convince him to leave that world and they all wake up together with the team.  Saito fulfills his part of the bargain and uses his connections to get Cobb’s criminal record cleared.   After arriving at his home, Cobb starts his totem top spinning on the table – his check on reality.  He is interrupted by his children coming into view and he rushes off to see them, leaving the top still spinning.  The credits of the film roll, not showing the audience whether or not the top fell, not knowing whether we were viewing reality or dream the entire time.

That is one of the great ways to finish off a film, to leave the audience questioning.  Some movies do it poorly, but Nolan did it superbly.  Your entire view of the main character, Cobb, changes with how you think that final totem spin resulted.  If the totem fell, then Cobb is in the real world and successfully overcame the justice system obstacle placed in front of him by his deceased wife.  His wife is the tragic victim who couldn’t accept that the dreams were not real and her memory haunts Cobb because he believes it is his fault.  On the other hand, if that totem keeps spinning, then Cobb is in a dream world, never having woken up from limbo, and is now as lost as his wife was.  You might even go further and think Cobb has always been in limbo since his wife left him.  Perhaps his wife’s rejection of reality was in fact a rejection of the dream world and Cobb is the one who has been stuck in limbo refusing to accept it as a dream.  Cobb’s wife is not haunting his subconscious but is in fact, alive and well in the real world and continues to join his dreams in an attempt to bring him back to reality.