Your mind is like an iceberg: Your conscious mind is only a tiny proportion of your minds total function, with 90% of processing done "under the surface" by your subconscious mind. We are all constantly taking in new information, processing it, and storing it every day. This is how we learn, and develop new patterns of behavior.
Our regular way of learning is through conscious, purposeful acts; for example, we study, read books, and study. However our subconscious minds absorb all kinds of information every day that we are not aware of, which also gets processed in our minds.
The theory behind subliminal messages is to take control of the information you send to your subconscious mind - to enhance your learning capabilities by targeting your subconscious mind with only specific, beneficial messages.
So instead of randomly absorbing information from our surroundings we can send positive messages to ourselves to aid learning only in areas we want to develop. We can harness the power of the 90% of our minds we normally do not get to be in control of.
Many people never truly take control of their minds and develop to their full potential. It is only a small percentage of society which excels past the "norm", and these individuals often are the ones who have learned to focus their minds and developed positive patterns of behavior to guarantee their success.
Subliminal messages can be used to accelerate learning, build confidence, increase motivation, relieve fears and phobias, resolve addictions, and much more. In fact whatever area you are looking to develop, or personal problem you are looking to overcome you can use the power of subliminal messages to help achieve your goal.