Alright kiddo, I'm going to explain to you what self-propelled artillery is in a way that is easy to understand.
Do you remember playing with toy cars that you could make move on their own? These cars were self-propelled - that means they could move themselves instead of you having to push them around.
Now imagine a big machine that is much, much bigger than your toy car. This machine is also self-propelled, just like your toy car! It's called self-propelled artillery.
So what does this machine do? It shoots big, heavy bullets called shells really far. It's like a giant gun that can move around all by itself. This makes it easier for soldiers to use because they don't have to move the gun around themselves.
Self-propelled artillery is very important in war because it can shoot from a safe distance, far away from any danger. But it's also really big and heavy, so it needs to be able to move around easily. That's why being self-propelled is so useful!
So there you have it, self-propelled artillery is basically a giant gun that can move around by itself. Pretty cool, huh?