CPR is a method of reviving a lost teammate in multiplayer.
When one of your teammates fall in battle, a letter will be above them (Y for Xbox and triangle for PS3). You will have to go near them and press the button. When the line crosses over the heart, you needed to hit the button again. The line will come back, and you will have to repeat again and again 5 times. The amount of health your partner gets back is dependent on how well your timing was on hitting the line on the heart.
Although this resurrects your teammate, you are still vulnerable to attacks while executing CPR. This can be very frustrating when you are in a clump of enemies because they won't stop attacking you just to watch you perform CPR on your friend. Thus, it is more advisable to kill all enemies before attempting to revive a teammate. However, sometimes it's impossible to revive your teammate in some areas because you'll get attacked, an example being the part in Industrial Castle with the stomping mechanism. If your teammate dies there, it will be hard to resurrect them since you'll keep taking damage from the trap. In this case, you may have to leave your teammate behind.