Bid price
The bid is the best price the market will pay to buy: it is the price at which you sell. It is always lower than the ask.
A quote shows two prices. The bid, on the left on most platforms, is what buyers are offering. When you open a sell (short) or close a buy, you are filled at the bid.
On MetaTrader, charts are drawn from the bid by default. A little-known consequence: a buy limit order, or a stop on a short position, triggers when the ask reaches your level, which the chart does not necessarily show. You can display the ask line in the chart properties.
The gap between the bid and the ask is the spread: your first cost on every trade.
See also