By sampling a small random number of patients, lake water samples, or bolts from the entire population of people, lake water, or bolts in the entire shipment, statistics can predict with a certain amount of certainty how closely the entire data set resembles the sampled population. Well, it is part of biology, part of chemistry, part of finance management, or just about any science, including business. Statistics is not math, but it uses mathematics. Statistics is a science of probability, a science that studies chance.
Chance is predictable to an extent.
So if you flip the coin 100 times, it should come up H about 50 times. It should come up T about 50 times. So chance is predictable, and can be assigned a number. The average, or mean, value of a set of numbers gives an indication of what value you would expect for a randomly taken sample other than the samples used to determine the average. The average is one measure of centrality for a data set.