Definition of resteal
A resteal is when you steal from a thief (to make a picture). Let's take a simple example. preflop, the players lie down until the button that opens. The button's raise will be named "steal" since it is in position to steal the blinds. We will therefore describe its raise of steal. Now, if the player in the SB position knows that the button opens too many hands and wants to defend himself, he will be able to raise the resteal, i.e. the reraiser. So, when a player in SB or BB position reraise a raise of cutoff or the button, we'll say he resteal (or 3bet, but the term resteal is more precise).
When do you have to resteal?
A resteal is interesting because it puts pressure on the player who steals the blinds. When a player steal on the button, he is in position. If you have call of the blinds, you are out of position without the initiative. It will be very difficult to play the hand. For this reason, resteal is possible. The player who steal will have a huge range and he will have to fold many hands. When he calls, the player who has resteal will have the initiative and will be able to represent a lot of strength. And if you make a resteal and your opponent simply call, you know that he won't have a premium hand, which will help to read him after the flop.