Definition of set mining
A player will "set mine" when he calls a preflop bet with small pair in hand in order to flop a set (a set consisting of a pair in hand). Let's say you are in button position with a pair of 5's. You open preflop to 3 big blinds. Your opponent in the small blind is a very tight player. After your raise, he 3-bets you. Here, you know that his range is very strong. Normally, with several hands, this would be a fold. But you know that if you hit, since he is strong, you can take his entire stack.
Set mining should be done for the right reasons
Let's go back to our example with our pair of 5's. The reason for the preflop call is our implied odds. We know that when we hit our set, our opponent has a very good chance of paying us because he has a lot of AA, KK, QQ, and JJ in his range. So, if the flop comes 5-2-9, he's going to bet a lot. After his bet, we can call. On the turn, let's imagine an 8. Naughty bets again and we call or push all in. It will be difficult for him not to lose his entire stack. We do set mining knowing that every time (or almost every time) we hit the flop, we win his entire stack.