Closed set

Imagine you have a toy box that has a lid on it. The toys inside are called points, but we don't want any points to escape outside the box. A closed set is like this toy box, where all the points are safely kept inside and are not allowed to go out. When we say a set is closed, it means that it includes all its "edge points" and doesn't let any new points come in or leave out. It's like a fortress that can't be penetrated by anything outside. So, when we talk about a closed set, we mean a set that is complete and finished, like a perfectly organized toy box with its lid on!