Something that your subconscious mind registers, without immediately triggering conscious recognition. e.g. subliminal advertising in a movie, just a few frames that might make you vaguely think something odd just happened but you can't say what - until you advance the movie frame by frame.
It is a verbiage (spoken words) or an image recorded to pass your conscious and address to your subconscious. meaning that when you watch or hear something and dont know it, you sometimes act on what you hear or see. its kinda hard to explain, i hope this helps.
not being aware of something