The bid price is simply the highest price that a buyer is willing to pay in any given market. Buyers mostly make bids, stating how much of a given asset they’re prepared to buy and at what price.
You can imagine all of the collected bids of all the buyers in a market as a list with the highest price at the top and the lowest price at the bottom.
When a seller comes along and sells a certain amount of whatever is being traded, that seller receives the price at the top of that list of bids.
If the seller is selling more than that top bidder is prepared to buy, then the remainder is sold to the next bid on that list.