The Krein-Rutman Theorem is a big idea that helps us understand some important things about certain types of math problems called linear operators. Linear operators are like machines that take things in, do stuff to them, and give something new out. The Krein-Rutman Theorem tells us that if we have a positive and compact linear operator, then there is always something called an "eigenvector" that is special to that operator. An eigenvector is like a superhero that always stays the same, no matter what the machine does to it.
Basically, the Krein-Rutman Theorem is like a secret code that helps us understand what happens when we use certain machines called linear operators. It tells us that if we look hard enough, we can always find something that stays the same, no matter how much the machine changes things around.