What are bid and the ask prices?
The price of a security (a stock for example) is basically determined by supply and demand, which, in the stock market, translates into bid and ask prices. All stock trading in the United States is based on the bid and ask system. The nature of this system is that sell orders are filled at the bid price (which is the highest price that somebody in the market is willing to buy at the security you want to sell), while buy orders are filled at the ask price (which is the lowest price somebody in the market is willing to sell at the security you want to buy). The difference between the bid and ask price is known as the spread. This is the amount market makers earn per each share on each trade. So, BID is the price one can SELL at, ASK is the price one can BUY at. The difference between the closing price of one day at 4 pm ET, and the opening price on the following day at 9:30 am ET is due to whatever happened during those hours that may have convinced owners of a particular stock to change
