The highest price currently available from a buyer. If you sell at market, you transact against the bid.
Full Explanation
The bid is one half of the market's two-price reality. It is the highest price at which someone is currently willing to buy. When you place a market sell, your order does not sell at a single abstract 'market price' — it meets the best available buy liquidity, which is the bid.
Most retail forex charts display the bid or something very close to it. That matters because the number you see on the chart is not necessarily the price at which a buy order would fill. The bid tells you where the highest resting buy limit orders are currently available. If sell aggression consumes those orders, the bid must relocate lower to find more buy liquidity.
Think of the bid as the top shelf on the buy side of the order book: the best price a seller can get right now, not a prediction of where price is going next.