The phrase "to keep in shape" means to maintain a healthy and fit lifestyle by engaging in regular physical activity, eating a balanced diet and getting enough rest. Full definition
A game of fetch, with the ball not being thrown a long distance, helps keep her in shape without over working her. (all-about-goldens.com)
It simply means that you need to keep yourself in shape by exercising frequently, cutting the excess fat and trim your waistline. (fitnessandpower.com)
I workout daily to keep in shape for the activities that I enjoy doing. (nativeamericandating.net)