Definition of restal
A resteal is when you steal from a thief (to make an image). Let's take a simple example. Preflop, the players have folded to the button which opens. The button's raise will be called a "steal" since he is in a position to steal the blinds. We will describe his raise as a steal. Now, if the player in SB position knows that the button is opening too many hands and wants to defend himself, he can restal, or reraise. So, when a player in SB or BB position reraises the cutoff or button, he is said to be resteal (or 3bet, but the term resteal is more precise).
When should we stay?
A remainderal is interesting because it puts pressure on the player stealing the blinds. When a player steals on the button, he is in position. If you call from the blinds, you are out of position without the initiative. It will be very difficult to play the hand. For this reason, the restal is a possibility. The player who steals will have a huge range and will have to fold a lot of hands. When he calls, the restal player will have the initiative and will be able to represent a lot of strength. And if you make a restal and your opponent simply calls, you know he won't have a premium hand, which will help him read it after the flop.