As for any acting exercise relaxation is the first thing to be aware of. If you want to understand the exercise, you'll need focus on what is going on. If you are not relaxed, you will have more trouble with the exercise.
Relaxation is about being in the moment, without being derived by other matters. All your muscles need to be ready to perform your role, your head can not be with your sick dad, or your schoolwork or any other thing than this part of the play.
Lets say you play Hamlet, but you also have an appointment after the show. It is you that has the appointment, not Hamlet! Thinking of the appointment, as important as it might be, will make Hamlet fail to convince the audience! And believe it or not: that will be noticed. You don't want that do you? Suppose the appointment was with a casting agent for your next role? Would he still be happy to cast you after this performance? No.
Relaxation is about letting all behind that does not apply to the scene or play you are in.
Now how can you create this state of mind?
With this acting exercise relaxation will become a natural habit. And you still will be able to keep the tension that is needed in the play.
To relax start lying on the floor.Close your eyes, so you will not be distracted by your surroundings.
Check your toes, are they relaxed? If not, relax them. Than your whole feet. Next your lower leg, your calves, do you feel tension? Loose the tension, and go to your knees.
Make sure that once you relaxed a body part that it stays relaxed. You can check it again, because sometimes when focussed on one part, in the other the tension returns.
Your thighs, your hips, lower back. Feel how your body connects to the floor. Make sure it feels comfortable.
Continue with your chest, breast, shoulders. It is a constant process of checking in this acting exercise, relaxation can be hard to achieve!
Next and also important, your neck, head, face. And what many people forget: your eyes, your mouth, your tongue.
Now everything is relaxed. Enjoy the feeling.
For a brief moment flex all your muscles as tight as you can and let go, relax, feel how you sink back on the floor, maybe you will notice even more contact as the first time.
More than any other acting exercise relaxation helps you to be aware of tension. If you do this every day, you will get more and more in control over you body, your muscles. This will help you to be relaxed on stage an choose the amount of tension needed in the scene, and let go again when the scene is over.
Make it a daily exercise for best results.