Submatrix

Okay kiddo, imagine you have a big square with a bunch of smaller squares inside of it. These smaller squares are called cells.

Now, let's say you want to pick some of these cells and make a new square with them. You can pick any cells you want, but they have to be next to each other in a certain way.

This new square that you make with the cells is called a submatrix. It's kind of like a part of a bigger puzzle that you take out and put aside to focus on just that part.

So, in short, a submatrix is a smaller square made up of adjacent cells from a larger square.