Rock
A rock is a very tight and passive player who plays only the strongest hands and rarely bets without them. The word is also used for a tight player in general.
The simple explanation
A rock waits for premium hands and folds the rest. When he bets, he has a strong hand, so you should usually fold. You can steal his blinds, since he folds too often.
A closer look
The rock is similar to a nit. Against rocks, widen your stealing range, bluff more when he shows weakness and fold to his aggression. The rock has a low win rate because he misses chances and gets few calls when he does bet. In some games, a 'rock' is also a token, such as a bundle of chips held with a rubber band, or the straddle in some games, which is a different, rare meaning.
A practical example
A player folds for an hour and then raises with aces. The others say, 'That's the rock; he's got a real hand.'
Full explanation
A rock is a very tight and passive player who plays only the strongest hands and rarely bets without them. The word is also used for a tight player in general.
A rock is a very tight, passive player.
His bets show real strength.
You can steal his blinds.
He is similar to a nit.