Empiricism is a way of finding things out by observing. Empirical facts are those which have been discovered by actually seeing them or perceiving them through one of your senses. For example, it is one thing to reason that bumblebees can't fly because they are too heavy for their wings, it is another to go out in the yard and observe them actually flying. Seeing them fly is empirical evidence that they can in fact fly, regardless of what your reason tells you. Empirical evidence trumps everything else. An advantage of empirical evidence is that others can verify your findings through the same observation— which makes it more convincing. Science emphasizes finding things out empirically.
Not using empiricism can lead to all kinds of mistakes. For example, Aristotle said that women had fewer teeth just because he assumed they would (he was one of the first sexists).
If he had tested this empirically by counting the number of teeth in a man and a woman, he would have realized his mistake.
Aristotle was also wrong when he said that heavier objects fall faster than lighter ones. Once again, he could have tested this empirically by dropping a heavy object and a light one off a building and having someone observe which hit first. But it seemed so obvious to him that he didn't bother with an empirical test.