Let's say you go to the store with your mom or dad and you see a toy you really want. The toy costs $5. You can't just take the toy without paying for it because that would be stealing, and that's not nice.
So your mom or dad gives the store owner $5 for the toy. This $5 is called the price. The price is how much money the store owner wants for the toy.
Now, let's say another store down the street also sells the same toy but for $10. You might want to go to the first store because it's cheaper. This is how price systems work. The prices of things help us decide where we want to buy things from.
Price systems also help make sure there is enough of everything for everyone who wants it. Let's say there are only 10 toys in the whole world and lots and lots of kids want them. The store owner might raise the price of the toy to $20 so that only the kids who really want the toy will buy it. This way, there will hopefully be enough toys for everyone who wants one.
So, in summary, a price is how much money someone wants for something, and price systems help us decide where we want to buy things from and make sure there is enough of everything for everyone who wants it.