Basically, there are a few known facts: (1) organisms die, (2) organisms reproduce, (3) the offspring are different from each other and from the parents, (4) the differences are more or less random, (5) dying is not random but a consequence of external forces, and (6) external forces are relatively constant. From these facts, it logically follows that over generations (or ofganisms reproducing ans dying), the living organisms will become more and more adapted to the environment. To avoid this long and detailed explanation, we just call this process evolution.
It's completely the same as e.g. the word "evaporate" - instead of saying that kinetic energy causes H2O molecules to become more and more excited, and slowly breaks the H bonds between molecules, so that they can move freely inspace (gas), we just say "water evaporates".