A redraw is the chance of a hand (that is usually currently behind) improving with more cards to come. It differs from a draw because it implies that the hand in question has already hit one draw in order to reach its current level, and hence would now need to re-draw to reach a higher level.
If you hold a made hand that can be improved, such as a set in Hold 'em after the flop, you have a redraw. This term usually indicates that you are behind but could draw out on your opponent. For example, in the case where a player hits a set on the flop but an opponent hits a flush, the set is said to have redraws because it could improve to a full house to beat the made flush.
Redraws go both ways, and occur on both the turn and the river in Hold 'em: In our previous example, if the player's full house is made on the turn, the holder of the flush might then have redraws to a straight flush.