A siege engine is like a really big catapult or sling-shot that people used a long, long time ago to try to break down walls or buildings during a battle.
Imagine you're playing with your toys and you want to throw a ball at your friend's castle, but their castle is too far away. You can't just throw the ball with your hand because it won't go far enough. So, you build a big slingshot or catapult with some sticks, rope, and rubber bands. This machine helps you launch the ball at the castle really far and hard, so you have a better chance of hitting it.
That's kind of what a siege engine is, but on a much bigger scale! Instead of launching balls, they launched huge rocks or even burning objects to try to break through walls and buildings during a battle.
The people back then were really clever and figured out many different designs for siege engines. Some were like big wooden towers that soldiers could stand on to shoot arrows or throw things at the enemy. Some were like big battering rams, which were giant wooden sticks that people would slam against the walls to try to break them down.
Siege engines were really important in battles a long time ago because they helped armies conquer enemy fortifications. But now we have much more modern weapons and techniques so we don't use siege engines anymore.