You're selling a bike through an ad and asking $120. A buyer offers $100. You settle on $110. That's the price of the bike — not the one you wanted, and not the one he offered, but the one you agreed on.
On an exchange it's the same, only with thousands of people. Some stand in a queue to buy, each with their own price. Others stand in a queue to sell. The best price among buyers is called the bid, the best price among sellers is the ask. These are the same two prices you see on the buttons: “Sell” is at the bid, “Buy” is at the ask. Both queues together are called the order book.