Callisthenics are exercises you do with your own body, not with heavy gym machines. Push-ups, sit-ups, squats, jumping jacks, holding a plank, pulling yourself up on a bar — that family of moves. The aim is to make you strong and healthy by using your body as the weight. Schools, armies, and people who train in parks all use this kind of work. Though actually, the word can sound a little old or formal now; younger speakers often just say bodyweight training, or they name the move. You will also see the spelling calisthenics (one L). That is the usual American spelling of the same word, not a different sport.