The highest price any buyer is currently willing to pay. When you sell, you transact at the bid.
Full Explanation
The bid is one of two prices that always exist simultaneously in the market. It represents the best available buy offer — what you would receive if you sold right now. The chart on most retail platforms displays a price at or near the bid, which means the number you are watching is the price buyers are willing to pay, not what sellers are asking. Understanding the bid is the foundation of understanding why your fills happen where they do.