In terms of adaptation, fitness refers to the ability of an organism to survive and reproduce in a constantly changing environment.
Adaptive traits are very much important in a constantly changing environment as they increase the chances of survival of a population in the environment.
For example, Darwin's finches adapted to develop different types of beaks in order to get nutrition in different environments. Long necks of giraffes are also considered as a result of adaptation which helps them get the food located high on trees.
