It's kind of hard to describe haha, but basically it's about a team who is able to use a hand held machine that allows them to tap into the subconscious of a sedated person. This team is then able to create a dream world of whatever they want with a dream architect (selected person who designs levels of the subconscious in the mind of the person who is being tapped into). They do so in order to retrieve information from whomever they are tapping into unknowingly. For example if someone was hiding 3 billion dollars they could create a dream world in which the sedated billionaire would reveal the location of the money, they call this extraction in the movie. Leonardo DiCaprio's character is wanted for suspected murder of his wife but can have the charges dropped if he does one more job, the only thing is that it's not extraction it's inception which is the planting of an idea or essentially creating false inspiration. So if you use the billion dollar example again but with inception you could plant the idea in the person's head and they would think they came up with it that they should just give you the 3 billion dollars because of whatever reason.
It was awesome and I loved it, so if you like complex movies it's definitely for you. But I have a feeling that if the average American watched it they'd be terribly confused and possibly bored.