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Stocks and other securities go up and down in price all day long. If you happen to order yours at the close of the market (at the end of business hours), you have an at-the-close order, and you'll generally be paying the closing price for the day (this can be bad news or good news, depending on what the closing price was).
It may be easier to think of it in terms of fast food: If Big Mac prices changed every minute, and you ordered one at the close (midnight at your local McDonalds), the burger flippers' clerk would scream your name at 11:59PM. Whatever the price for BigMacs was at that moment would be the price you'd be charged.