What does it mean by playing your opponent? Some of the things to consider:

• Play more hands against players with loose starting hand requirements and fewer hands against tight players.
• Play more hands against players who play badly after the flop.
• Don't bluff very often against loose players and play aggressively against tight players
• Induce bluffs against loose aggressive players. Induce calls against tight players
• After the flop, tend to call, raise and re-raise your borderline hands against aggressive, tricky opponents and fold against rocks who never bet or raise without a strong hand.
• Note that the playing style of your opponent pre-flop is sometimes different to that after the flop.
• Generally try to sit to the left of maniacs and to the right of tight players.
• Mix up your strategies based on the flow of the game to keep your opponents off balance.            

These are only some of the things you might consider.