So when you do something on purpose it means that you meant to do it. You did it intentionally. You did it deliberately. For instance you could say that, "He set the fire on purpose." That would mean that the person decided to set the fire and then they did it. So it's the opposite of having something happen "by accident". When something happens by accident it's something that you didn't choose to do. So hopefully that makes sense. You might have to look up the word "deliberately" and "intentionally" and that will help you understand the word, "on purpose".