Subliminal messaging is using certain techniques to get messages to people without them being consciously aware of it. For example, pictures flashing really quickly or backmasking (putting things backwards in songs).
People believe that the person won't consciously notice it, but that their mind will perceive it and get the message, then they will act on the message without even realizing.
Subliminal messaging is controversial and whether or not it does anything at all is still debated. Last I read, scientists found that it does slightly influence people's decisions on really minor things (what soda should I get?) if they can't make up their mind, but subliminal messaging isn't even close to actually being able to put thoughts into people's minds or to control their behavior in a dramatic way.
edit: however, the more you're exposed to something, the more you're likely to like it, so putting brand names in the backgrounds of things, or even possibly subliminal messaging, may make you more partial to the brand long term. But the influence is nowhere near as powerful as people think or expect.
People are generally accused of doing it for advertising purposes, or they're accused by conspiracy theorists of trying to brainwash people into doing things.