The word "subliminal" has a Latin origin, literally meaning "below the threshold." This refers to the mental threshold of conscious awareness, which is composed of everything that we perceive while also being aware of our perception. Because our five senses are constantly receiving sensory data, our minds receive a vast amount of information outside of the focal point of our awareness. This information is referred to as being subliminal, or subconscious.
The appeal of subliminal words or messages is that they can affect your thoughts or opinions without your being aware of their presence. This idea has its origin in various theories of psychology and philosophy, which say that the human mind is affected to some extent by all of the information it receives, even subconscious information. Since subliminal words remain in your subconscious mind, they can convey an idea to you without you ever actually thinking about the idea, as you normally would when reading.
Subliminal information can be conveyed to a reader or observer in a number of ways. A subliminal image can be conveyed to an observer by incorporating hidden images into a larger image, or by arranging smaller parts of an image so they spell a word or form a larger image that isn't directly noticed. Subliminal messages can also be hidden into audio tracks, as when certain words are emphasized at a timed increment so the mind subconsciously pairs those words together. This can be used to form an altogether different meaning in the listener's mind, as opposed to the audio that is consciously heard.
There has been much debate on the legality and ethics involved with subliminal advertising. Nevertheless, you can see various subliminal methods at work in virtually all advertising. Attributes such as font, color and layout are generally planned out by advertisers to set a subconscious mood in the mind of the person viewing the ad. However, using hidden, subliminal words in an advertisement is rare, since it is generally viewed as deceptive and is less effective than the subtler methods.