Fitness is a set of sports exercises that are practiced in a gym. A fitness training can be aimed at training certain muscles or muscle groups, or to improve endurance and condition.
With fitness you focus specifically on training and maintaining your body. Sometimes fitness is supportive of another sport that you get better at by being fitter. In specific cases it is important to recover after a period of illness. And often fitness is a way to rebuild everything a bit after a period of neglect of your body.