A rock is a common term for a strong tight player. This means they only play good hands, and most of the time will win them. Rocks do well in poker because they lose very few chips to silly bets.
Rocks are probably the easiest player to play at a poker table. You know they are tight and if they bet they probably have it, so unless you have the nuts just fold. There will be times when a rock is betting into you when you have the nuts, this is when you take advantage and beat them. Remember if you do bet a rock make sure you have it because if you have the second hand out you can almost guarantee they have the best.