A nit in poker is a player who plays a very tight style and does not get involved in big pots unless they have the nuts or are close to it.
For example, a poker nit will play only a small percentage of the range they’re dealt, and when they do get involved, they’ll usually play fit-or-fold postflop. If they open with Ace-King and don’t hit the flop, they’ll be ready to give up without too much resistance. They are not likely to float your bets or isolate other players.
Playing against nits can be frustrating as they won’t give you as much action as maniacs or LAGs, and they are the ones who are most likely to hit and run. However, these players are not hard to beat. When they start betting and raising aggressively, you should usually fold. In most other circumstances, you can win pots off of them without much resistance.