Okay kiddo, have you ever heard the word "spin-off"? It's when something new is created out of something else. When we talk about a university spin-off, it means that something new has been created as a result of the work done at a university.
Here's an example: Let's say a group of professors at a university do some research on how to make a new kind of medicine. They work really hard and eventually figure out how to make the medicine. Now, instead of keeping it to themselves, they want to share it with the world so that people can use it to get better. They can't just go into business and start selling the medicine themselves, though. That's where a university spin-off comes in.
A university spin-off is a new company that is created to take the ideas and inventions that are developed at a university and turn them into real products that can be sold in the marketplace. Think of it like a baby bird leaving the nest - the university has nurtured and cared for this idea until it's ready to leave and start a new life on its own.
The spin-off company will usually be started by the professors who did the research, along with other business people who can help them make the new product and get it out into the world. The company will usually be based near the university, because it's where the ideas and people who helped create the new thing are located.
So, to sum it up for you, a university spin-off is what happens when a new company is created to turn an idea developed at a university into a real thing that people can use in their everyday lives. It's like a baby bird leaving the nest and starting a new life on its own, with the help of some grown-ups who know how to take care of it.