A 'set' is what poker players call making three of a kind when holding a pocket pair.
For example: If we hold the following hand:
The flop runs out:
In this example we can see that we have flopped a set.
Set-mining is when a player calls pre-flop with a pocket pair with the intention of folding on the flop if they do not flop a set. We must understand the maths behind flopping a set to identify when it is profitable to do so. Mathematically we flop a set 1 - 7.5 times we see a flop. We also have to take into account that we will not win all of our opponents money when we do eventually flop a set. We should therefore be looking to get a minimum of 15-1 in implied odds when we are set-mining.
We should remember what implied odds are. Implied odds refer to the money that is left in our opponents stack that we could potentially win. Therefore if we need to call $5 with 44 to set-mine, we need to be against an opponent who has a stack of at least $5 x 15 = $75. If not, our call is unlikely to be profitable.