So, have you ever been to a store where they sell things for other people? Like someone gives the store some clothes or toys, and then the store sells them for that person? That's called consignment!
It's kind of like borrowing toys from your friend. You take care of their toys and play with them, and when you're done, you give them back to your friend. But in this case, the store will sell the things for the person who gave them to the store, and then give them some of the money they make.
So, let's say your friend gives the store a dress to sell. The store will put the dress on a rack with a price tag on it. When someone buys the dress, the store will take some of the money, usually around 50%, and give the rest to your friend. Your friend still owns the dress until it gets sold, and they can take it back if they want to.
It's a good way for people to sell things they don't need anymore, and for stores to have more things to sell without having to buy them all themselves.