Overspill is what happens when there is too much of something in a certain place, and it needs to go somewhere else.
Think of it like when you're playing with water in a bathtub. If you pour in too much water, it starts overflowing and spilling out of the tub. That's overspill! The water had nowhere else to go, so it spilled out of the tub and onto the floor.
The same thing can happen with lots of different things, like people or animals or even ideas. For example, if too many people want to go to a concert, there might not be enough space inside the building for everyone to fit. So some people might have to wait outside (or go to a different concert) because the space inside is already full – that's overspill!
Basically, overspill is when there's too much of something in one place, and it needs to go somewhere else to fit properly.