Closed immersion

Imagine you have a castle and a moat around it. The castle represents a bigger space we call a "scheme" in math. The moat around the castle represents a smaller space called a "subset".

A closed immersion is like having a bridge that goes from the subset (moat) to the scheme (castle), but there are gates on both sides of the bridge that prevent anything from leaving the subset or entering it without first passing through the gate.

In math, a closed immersion is a way of mapping a smaller space (subset) onto a bigger space (scheme) while maintaining the special properties of the smaller space, such as a closed set of points. Think of it as a restricted pathway where you can only go through the gate to get in or out.