How is a stock price determined at the very beginning when markets open?

When the market closes the stock has a fixed price. We have trading during closing hours and that affects the price as well.
But I have seen multiple times on stocks such as AMD that when I open an order to buy before the opening hours, I have to pay more than the price during closing hours. I don't understand how can the price jump when I sent my order a few minutes before the market opens.