Puberty is the time when your body begins to develop and change as you move from kid to adult. We're talking about stuff like girls developing breasts and boys starting to look more like men. During puberty, your body will grow faster than at any other time in your life, except for when you were a baby. It also happens because it helps for reproduction in humans. This way we can have more babies to run around the world but its good so we always have another generation after us.
I hope I helped!