The bid price is the price at which one can sell a given size of an instrument at a given point in time, i.e. the bid is a price at which a buyer is willing to buy a security. This is usually less than the Offer or 'Ask' price
Vodafone shares are quoted at 100 (50,000)- 100.5 (100,000) The bid price is 100 and 50,000 shares are available at this price.
 See also