Schur complement

Schur complement is a way of taking a larger matrix (like a big grid of numbers) and breaking it into two parts. Imagine that you have a rectangle with 10 columns and 10 rows of numbers. You could split the rectangle into two squares, one with five columns and five rows, and one with the other five columns and rows. The Schur complement lets you do this and then make calculations for both squares separately by using some of the numbers from the other square. It makes calculations easier and faster.