A metabolic or physiologic adjustment within the bodies cell and/or or tissues, of in response to an environmental stimulus (for athletes – the training load) resulting in the improved ability to cope with its changing environment.
A body grows and becomes stronger and faster AFTER it has adapted to a training stimuli. So the amount of rest and recovery you give your body is just as important as the training itself. Without sufficient recovery, over time, your body won’t be able to absorb the training load and make the adaptations to increase performance.
Further reading here