When looking at stock quotes, there are numbers following the bid and ask prices for a particular stock. These numbers usually are shown in brackets, and they represent the number of shares, in lots of 10 or 100, that are limit orders pending trade. These numbers are called the bid and ask sizes, and represent the aggregate number of pending trades at the given bid and ask price.
For example, assume we get a stock quote for XYZ Corp. and we see a bid of $15.30 (25), and an ask of $15.50 (10). The bid price is the highest bid entered to purchase XYZ stock, while the ask price is the lowest price entered for this same stock. As you can see, there are numbers following the bid and ask prices, and these are the number of shares that are pending trade at their respective prices. At the current limit bid price of $15.30, there are 2,500 shares being offered for purchase, in aggregation. The aggregation is for all bid orders being entered at that bid price, no matter if the bids are coming from one person bidding for 2,500 shares, or 2,500 people bidding for one share each. The same is true for the numbers following the ask price.
If these orders are not carried out during the trading day, then they may be carried over into the next trading day provided that they are not day orders. If these bid and ask orders are day orders, then they will be canceled at the end of the trading day if they are not filled.
The spread between the two prices is called the bid-ask spread. If an investor purchases shares in XYZ, they would pay $15.50. If this same investor subsequently liquidated these shares, they would be sold for $15.30. The difference is a loss to the investor.