Sounds like something from outerspace doesn't it sometimes? They've been around since caveman paintings you could say.
It's about communication.
We need to understand the concept first of all of the conscious and unconscious (synonymous to subconscious) parts of the human mind.
The conscious mind is the part of you that "thinks".
It is what you are consiously aware of as going on in your mind.
Compared to the subconsious mind, it is very basic.
It can hold up to 6 or 7 things at once usually before getting distracted or confused and often it is a source of much frustration.
The unconscious mind however is the part of the mind that does the things we take for grantid generally.
This includes but is not limited to walking, breathing, circulation, right down to the molecular processes that are going on within the cells.
You don't have to think consciously "right now, in this particular cell in my body, I am going to manufacture a protein that tells another part of the cell to manufactaure another protein which will cause an increase in glucose uptake through the cell membrane".
Of course you don't and that is why our creator has very cleverly given us an unconscious mind to have these things taken care of for us.
Also, the unconscious can take responsibility for us in terms of repeated behaviors.
This can work for or against us.
When you have consiously learned to drive, you eventually turn it over to your unconscious mind and can think consiously about several things at once, even hold a conversation whilst listening to music as you drive, taking back conscious control only temporarily and when necessary, eg a hazard.
The great thing about the unconsious mind is that we can operate it consiously if and only if we remember how and as a result be able to tap it's resources and create wonderful and seemingly impossible experiences in our lives.
This is where subliminal messages can come in very handy.
Firstly, we bypass the conscious mind and give a clear instruction, direction or message to the unconscious mind.
The unconscious mind does not discriminate - reasoning is left to the conscious mind.
So if we bypass the conscious mind, and give a command to the unconscious mind, we are much more certain to achieve that which we want.
This is the power of the subliminal message in advertising, art and other mediums.
We can directly connnect with the subconscious part of ourselves which is comparably infinitely more powerful, capable and aware than the conscious mind.