If you want to understand 'Inception' (movie) when you watch it for the first time itself, then you need to know the below prerequisites (don't worry, no spoilers ahead). After I watched the movie and discussed it with a couple others, I realized that many would have understood and further enjoyed the movie if they were aware of a few well known concepts. Besides this, you will obviously have to pay close attention to the dialog!

So here are some 'Prerequisites to watching Inception':

1. The sounds and movements happening around you while u sleep, may manifest in some form in your dream. E.g. You may dream of gunfire if a loud washing machine is running nearby and so on...

2. Sigmund Freud divided mind into the conscious mind or ego and two parts of the unconscious: the id or instincts and the superego. While we are aware and in control of our conscious mind, we don't have such awareness of the subconscious (unconscious mind). E.g. A person who is lying may subconsciously: attempt to divert attention away from the lie, avoid eye contact, fidget and so on.

So its possible to 'extract information' to a certain extent from a person under hypnosis, the movie takes this a step further. How? Well you will have to watch the movie to find out!

3. Current technology can monitor brain wave activity to a certain level. Theoretically, in the future it may be possible to project signals directly into the brain itself. Something like the cartoon Jetsons, where Elroy selects a dream and the dream machine placed over his head will project that dream into his mind. So it may be possible to (literally) 'enter' into someone else's dream.

4. Lastly, we can control and alter our dreams while we are still dreaming. This is know as 'Lucid dreaming' which is very cool, so read more about it!