What is a raincheck?
A raincheck is a "coupon" for lack of a better word that you get at a store when they are out of an item that you want.
How do you get a raincheck?
When you go to checkout, tell the cashier or customer service desk that you need a raincheck for ________ (whatever you needed it for). They will fill out the paper and put all of the specific details.
How do you use the raincheck?
When you go back to the store and find the item you wanted, you present the raincheck at checkout and they will have a manager override the price.