"Just in time" is a way of doing things in a business. It means that instead of keeping a lot of things in a big storehouse, the business only buys what it needs right now.
Think about when you are playing with your toys. You might have a big box of toys, with lots of different things in it. But when you want to play with your blocks, you only get out your blocks. You don't need all the things in the box all at once.
The same thing happens in a business. The business only gets what it needs right now, so that they don't have to keep a lot of things around that they might not need for a long time. This saves the business money and helps them to be more efficient.
For example, imagine a pizza restaurant. If they make too many pizzas, they might end up throwing some away because they didn't sell. That's like having too many toys in your box that you never play with. But if they only make enough pizzas to sell, they save money and don't waste anything.
So "just in time" is a smart way for businesses to do things, by only getting what they need when they need it.