The ask price is simply the lowest price that a seller is willing to accept in any given market. Sellers set the asking price by stating at what price they’re prepared to part with given security and how much of it they’re prepared to sell.
This ask side of the order book is essentially a list of all the ask prices of all the sellers in a market with the lowest available price at the top. When a buyer wishes to purchase a given amount of the asset being traded, they receive the price at the top of the list of asks.
If the buyer is trying to purchase more units than the top asking price has available, then the remainder of the order is filled by the next ask price on the list.