Radiotherapy uses high-energy rays, usually x-rays and similar rays (such as electrons), to treat disease. It destroys cancer cells in the area that’s treated.
Normal cells can also be damaged by radiotherapy. They can usually repair themselves, but cancer cells can’t.
Radiotherapy is carefully planned so that it avoids as much healthy tissue as possible. However, there will always be some healthy tissue that’s affected by the treatment and this will cause side effects.