I think it's also a too, because working your body harder than usual in order to improve how it functions. Exercising at a higher level forces the muscles and systems in your body to eventually adapt. When you overload your muscles, they contract more, which increases their size. Your muscles' cells will also develop additional mitochondria, helping them provide more power in the future.
And FITT is the principle for an exercise prescription to help participants understand how long and how hard they should exercise.
