Rock refers to a player who is very tight and doesn't take many risks. They're usually only in a hand if they have a good chance of winning. Nit refers to a player who is even tighter than a rock. They're usually only in a hand if they have a great chance of winning.
Pluribus's self-play produces a strategy for the entire game offline, which we refer to as the blueprint strategy. Then during actual play against opponents, Pluribus improves upon the blueprint strategy by searching for a better strategy in real time for the situations in which it finds itself during the game.