Kidney stones are like little rocks that can form inside your kidneys. Your kidneys are located in your belly, and they help your body get rid of waste materials and excess fluid by making urine. Sometimes, the substances in your urine can stick together and form hard crystals, which can grow bigger and become kidney stones. These stones can be very painful and can cause an ache in your belly or back. But don’t worry, doctors can help you pass them out of your body.