Definition of set mining
A player will make a "set mining" when he calls a pre-flop bet with a small pair in hand in order to make flopped a set (a three of a kind bet consisting of a pair in hand). Let's say you are in the button position with a pair of 5's. You open preflop to 3 big blinds. Your opponent on the small blind is a very tight player. After your raise, he 3bet you 3bet. Here you know that his range is very strong. Normally, with several hands, it would be a fold. But you know that if you hit, since he's strong, you can take his whole stack from him.
You have to make a set mining 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 touch our set, our opponent has a very good chance to pay us because he has a lot of AA, KK, QQ, and JJ in his range. So if flop falls 5-2-9, he's going to bet very often. After his bet, we can call. On the turn, let's imagine an 8. Naughty bet again and we call or push all in. It will be difficult for him not to lose his entire stack. We make a set mining knowing that every time (or almost every time) we hit the flop, we win his entire stack.