The hot receiver is a pre-designated receiver who must be ready to catch a ball immediately after the snap in the event of a blitz.
If the quarterback recognizes prior to the start of the play that the defense is going to blitz, he will audible to the hot receiver. This means that as soon as the ball is snapped the quarterback quickly throws the ball to the receiver. This is done for several reasons. The first is that on a blitz the quarterback needs to get rid of the ball quickly, otherwise he will be sacked. The second reason the hot receiver is so effective is because with so many defenders blitzing, this leaves "open space" for the receiver to run with the ball and make yards after the catch.