A gym is a place where people go to exercise and get fit. It's kind of like a big playground for adults, but instead of swings and slides there are different machines and equipment to help you move your body and get stronger. For example, there might be big heavy weights you can lift up and down or machines that allow you to run indoors.
Some gyms will also have classes you can take with other people like yoga, Pilates, or kickboxing. These are like special games where you follow along with a teacher who tells you what to do.
It's important to warm up your muscles and stretch before using any of the machines or taking classes to make sure your body is ready to exercise. And when you're done, it's also important to cool down and stretch again to prevent injuries.
Overall, the gym is a cool place where you can have fun while being active and healthy.