Three of a kind made with a pocket pair and one board card.
The board has no pair, so it is hard for opponents to see. A pocket pair flops a set about 1 time in 8 (11.8%). That is why, when you play a small pair, what you can win when you hit matters.
At the table
Holding 7♠ 7♦ on a flop of 7♣ K♥ 2♠ is a set of sevens. Three of a kind made with one hole card and a pair on the board is called trips instead.