When you go shopping with your mom or dad, you know you have to pay money for the things you buy. That's like a tax that the store charges you, and they have to pay that tax to the government.
Sometimes, the government wants to double-check if the store is being fair when they add that tax. That's called a sales tax audit. It's like a grown-up coming to the store to count how much money they made and how much tax they charged people.
If the government thinks the store didn't charge enough tax, they might ask the store to pay more money. Or, if the store charged too much tax, they might give some money back to the people who paid too much.
Just like when you're doing your homework and your teacher checks to see if you did it right, the government is checking to make sure the store is doing things correctly.