Okay kiddo, so a push dagger is a special kind of knife that is designed to be held in your hand kinda like a pencil. You push it forward like you're punching someone, and the blade comes out of the top of your hand.
Now, this type of knife is actually pretty dangerous, so it's not for playing with. In fact, it's illegal to have in some places, and you should always be careful when handling it.
The reason people use push daggers is for self-defense. Maybe someone is trying to hurt them, or they need to protect themselves from an animal. If someone attacks you, you can use the push dagger to try to stop them and defend yourself.
But again, just because it's a tool for self-defense doesn't mean everyone should have one. It's important to be responsible and obey the law.