Underplay is a casino term most often used in poker; it refers to the way someone plays out their hand, especially if they have a very strong hand and good chances to win.
For example, when a good poker player has a favourable hand, they will try to play it slow, which means betting small or taking time before betting or calling, as if to pretend they are thinking about whether they should continue playing.
After all, if you go all-in immediately, players will either think that you are bluffing or will know that you have a very strong hand. In that case, chances are that no other player will partake in the pot. In other words, even if you win, you won’t be winning much.
Therefore, players try to lure others and increase the size of the pot by betting small and letting the hand progress, making others think that they have good chances of winning the hand and making them bet more into the pot.