Something that is adaptable can change to fit its environment, whether that environment is natural or social. The level to which a species is adaptable to changing surroundings is an important factor in the study of evolution.
Let’s look at the Latin word adaptāre, from the root aptus "fitted." The ad- means "to, while the aptare means "join." Since 15th-century medieval French, the word adapt has come to mean "to fit in or adjust." So something adaptable is something that "fits in or adjusts." An example of an adaptable creature is the arctic fox, whose fur changes from dirt-dark in summer to snow-white in winter, to better blend in with its surroundings.