Maximum heart rate
Maximum heart rate is the highest number of beats per minute your heart reaches during an all-out effort. It is largely determined by age and genetics, doesn't rise with training, and is one way to set heart rate training zones.
Age formulas
The well-known 220 minus age formula is a population average with a wide spread — individuals are often 10 or more beats above or below it. The Tanaka formula, 208 minus 0.7 × age, fits adults a little better, but any formula is only a starting point.
Finding your own
The highest heart rate you see at the end of a hard race or a hill-repeat session is a practical estimate. Only attempt a maximal test if you are healthy and used to hard training. Many coaches prefer threshold heart rate to max heart rate as the anchor for zones, because it is easier to test and more closely linked to performance.