Maximum Heart Rate is a measure of the maximum number of times your heart can safely beat per minute (BPM).
This figure will decline with age.
Your heart rate can be measured by using two fingers on your pulse (either at the neck, or on the wrist).
Use a watch or stopwatch and count the number of beats over 15 seconds - then multiply by 4. This will give you your heart rate BPM.
Some exercise machines will calculate heart rate (although this can sometime be inaccurate). The most accurate way of measuring heart rate is to use a Heart Rate Monitor (uses a chest strap or wrist strap).
Like so many techniques in fitness and fat loss, there are diverse opinions and conclusions.
However most people agree that a heart rate of 60-70% of your maximum is best when you are aiming to burn fat and are on a reduced calorie diet. At this zone more calories are burned from fat (during the exercise).
However - higher heart rates burn more calories in a shorter period of time, and provide a metabolism boost (afterburn) but during the exercise activity fewer calories are burned from fat.
It can be very confusing, but always remember - some exercise is better than no exercise so do not let complexities prevent you from getting out there and getting your heart rate up.
This involves short bursts (30 seconds to 1 minute) of exercise at 90% (or even maximum) heart rate, followed by a 1-2 minute recovery period.
However if your aim is simply for fat loss - use the 60-70% zone. There are some that fear HIIT training can burn too much muscle when on restricted calories - but this is a contentious issue.
Programs such as the popular P90X Plan advocate this method for losing fat and building muscle mass.
The tanaka method is essentially a formula of 208 - (0.7 x age).