The highest price a buyer is currently willing to pay 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 bid is the best price on the buy side of the order-book. When you sell with a market-order, the bid is roughly the price you get.
It matters because every trade has two prices, not one. If a stock shows a bid of $49.98 and an ask of $50.02, a seller receives $49.98 while a buyer pays $50.02. The gap between them is the bid-ask-spread.
Example: you hold 100 shares and the bid is $49.98. Selling at market gets you about $4,998 before commissions, not the $5,000 you might expect from the last printed price.