What is my max heart rate? And why is this important?

Your max heart rate is 220 minus your age. So if I am 50, my max heart rate would be 170 (220-50). So, theoretically, my heart rate should not go above 170 beats per minute (BPM).
This is an important number to know because it gives you a general sense of how hard you should be working out. So, if you are wanting to keep your heart rate around 78% when you run, then you would multiply 170 x 78% (.78). This would give you 132.6 BPM as your target heart rate for that run. It's a good general tool to use for an effective workout.
With that said always remember that these equations are often based on a one-size-fits-all theory. This doesn't mean the equation isn't accurate - but you may need to leave a little room in there for individuality.
The good thing is that your body has a built in mechanism that will be triggered if your heart rate gets too close to your max heart rate for too long. You usually will have so much burning in your legs (lactic acid build up) or you will begin to feel ill. That is your body's way of saying, 'Hey! Slow down!' It's good to listen to this....
Also - if you are not so into doing all this math - purchasing a heart rate monitor is a quick and easy way to keep track of your heart rate.