Idea tranfer is taking a concept from an area where it is known to work and applying it to a new problem.
I use a small version of idea transfer every day. As I apply ideas from books to my life, I'm transferring the idea to a new problem: me.
But the beauty of idea transfer comes from using ideas in entirely new contexts.
For example, the theory of contraints. If your assembly line is bottlenecked by a machine or job, no amount of optimization elsewhere in the factory will resolve the problem. In fact, such optimization would make things worse. Find the bottleneck. Fix it specifically. Repeat.
I learned how the theory of contraints proved itself in factories, and realized that it applied to the car wash I ran as a kid. Or to the software I was developing.
Keep in mind the assumptions of the original idea. Optimizing the output of my car wash doesn't help if the cars aren't clean.