The lowest price a seller is currently willing to accept for an asset.
The bid-ask spread. Buy orders sit below, sell orders above, and the gap between the best bid (50.01) and best ask (50.04) is the spread you pay to cross. Bar length shows the size resting at each price.
The ask, also called the offer, is the best price on the sell side of the order-book. A market-order to buy fills at roughly the ask.
Buyers pay the ask and sellers receive the bid, so the ask is always at or above the bid in a normal market. The difference is the bid-ask-spread, which is a real cost of trading even when commissions are zero.
Example: the ask on a futures contract is 5,000.25 and the bid is 5,000.00. A buyer at market pays 5,000.25 and is immediately down one tick if they had to sell right back at the bid.