Subliminals are affirmations or suggestions that are placed just below the normal auditory threshhold, that is just below the level of normal hearing. The theory is that your subconscious mind can still hear them, thus bypassing the conscious mind. Another way is to transform the affirmations and suggestions so that they play at a frequency of 12,000hz or higher, which is beyond the normal threshhold of human hearing. But some believe that the subconscious can still hear them and thus make use of them. 
There is conflicting evidence as to whether they really work or not. We have decided to include them in select products due to repeated requests that we do so.