Core terms
Bid
The highest price a buyer is currently displaying for a stock, together with the number of shares wanted. Selling "at the bid" means selling immediately to that displayed buyer.
A bid is a standing offer to buy. Market makers, other traders and algorithms post bids at different prices; the highest one is the best bid. If you place a market order to sell, it fills against the best bid first, then the next one down if the size runs out.
The displayed bid is not the full picture. Hidden orders, odd lots and orders on venues your data feed does not cover can sit at or near the same price.
What traders should verify
Check the bid's size as well as its price. A bid of 100 shares can disappear the moment you need it.
Do not conflate
A bid can be cancelled at any time before it trades. Quote displays differ by data feed.