Selling is a professional wrestling slang term that describes the act of making wrestling look believable to the audience.
When one wrestler performs a wrestling move, it is the opponent's duty to react to the move as if it had been done for real. This helps preserve the illusion of kayfabe. The most common example of this is when a wrestler performs a punch. The attacking wrestling usually stomps his foot at the time of near-impact while the receiving wrestling mimics the effect of being punched. When performed properly, it suspends the belief of the audience.
If a move is badly sold, it is usually referred to as a botch.
Whenever a wrestler doesn't react or acknowledge a move performed on him or her, it is referred to as a no sell. This may be for a storyline purpose (i.e. a wrestler is so much bigger and/or tougher that the move didn't work on them), or it's a botch.
Whenever the reaction to a move is disproportionately stronger than the likely effect it is known as an oversell. Typically some jobber to the stars are known for overselling as it makes their opponents look particularly powerful and dangerous. Curt Hennig is a good example of someone who would oversell. Sometimes overselling is done for comedic effect, like Ric Flair or Greg Valentine's face flop.