Motile organisms can move. If you put a drop of pond water under a microscope, you might see motile creatures darting around.
Biologists are interested in whether or not very small organisms are motile. Some doctors are concerned with how motile a person's digestive system is, or how efficiently food is moving through it. The movement itself is called motility, and both words come from the French motilité, from the Latin motus, or "motion," with motile copying the form of mobile.