A siege is when some bad people try to take over a castle or a town. Imagine you are playing with your blocks and you have a really cool tower. Your friend wants to take your tower for themselves, but they can't just knock it over. Instead, your friend and their friends set up outside your tower and won't let anyone go in or out.
Now, the people in the tower can't go get food, water or other things they need. They can't go out to play or to see their friends. They can only stay in the tower and hope they have enough stuff to last until the mean people go away. This is how a siege works.
In real life, a siege works the same way. Enemies surround a castle or a town and stop anyone from going in or out. They might even attack the people inside if they try to come out. This can last for weeks or even months, and it's very hard for the people inside to survive.
The people inside might try to send messages for help or food, and some brave soldiers might try to sneak out and bring help back. But most of the time, they just have to wait until the mean people get bored and go away.