A founder is like the person who has an idea for something new and makes it happen. Like when you build a sandcastle at the beach, you have an idea in your head of what you want it to look like and you start working really hard to make it happen. That's kind of what a founder does, but instead of a sandcastle, they might have an idea for a new kind of toy or a way to help people buy groceries online.
The founder doesn't usually do everything all by themselves though, because building something new can be really hard work. So they might ask their friends or other people who are good at certain things to help them make their idea a reality. Like if you wanted to build a really big sandcastle, you might ask your friends to help you carry buckets of water or dig in the sand.
If the idea is really good and people think it will be helpful or fun or make their lives easier in some way, then the founder might be able to make a business out of it. That means they can make something that other people will pay money for, and then they can use that money to keep making the thing even better. But it all starts with the founder and their idea, like when you start building a sandcastle with a big vision in your head of what you want it to be like.