Let’s say you’re trading stocks. The first price (usually one on left) is called a “bid”. This is price at which market is offering to buy stock. If you sell your stock at market, this is price that you’ll get. The second price (usually located on right) is called “ask”. This is price at which market will sell you stock. If you submit an open order to buy shares at market, you will get them for ask price. Another element that comes into play sometimes is size of bid and ask. Usually, there’s an order size that comes with bid and ask. If that size is exceeded then price will usually change – and generally, that small price change will move slightly against you since you're creating a demand for that stock.