When you enter an order to buy or sell a stock, you see the bid and ask for a stock and some numbers. What are the bid and ask, and what do those numbers mean? One, the bid, is what you need to know when you are selling a stock. The other, the ask (or offer) is what you need to know when you're buying. But you also need to know those numbers. Here's how it works:
If an investor looks at a computer screen for a quote on the stock of XYZ, it might look something like this: Last: Bid: 20 Ask: 20 1/4 BSize: 12 ASize: 5. The translation: the stock of XYZ is being bid at $20 a share and offered at $20 1/4 per share. There are 1200 shares bid for and 500 shares offered. If you are looking to sell stock, now you know there is a firm willing to pay (that's the bid side of the market) $20 for your stock, and that you could sell at least 1200 shares of stock at that price. Those are the two parts of the bid side of a market on a stock: the price and the quantity of shares at that price.
If you are looking to buy XYZ stock, you would have to pay $20.25 and could buy at least 500 shares of stock. Again, there are two parts to the ask side of the market: the price at which you can buy stock and the amount of stock you can buy.
When you look at a quote for a stock, it's only good for the time at which you check it. The bid and ask and the sizes for each side change constantly. If you were to check back in two minutes and you'd like to sell your XYZ at $20, the $20 bid may not be there because the stock may have moved up or down in that time frame. So each time you trade, you'll need to check the bid and ask to see where your particular stock is trading.
Whenever you enter an online trade, a "live" quote will be shown so you'll know where the stock is trading and what to expect if you buy or sell your stock. However, be aware that the stock can move very fast and that you may not get the price shown on your screen. That's because by the time your order is sent to the floor to be executed, the bid and ask may have changed because there was an order that came in ahead of yours and wiped out the bid or offer. Then the stock moves to a new level and the bid and ask will be different from what your screen showed when you entered the order. This doesn't happen very often, but it does happen. And when investors enter their market orders (meaning they will buy or sell stock at the market, no matter where the market for the stock is), look at the bid or ask, and then see their execution price is different from the stock prices they saw, they have to realize that stocks can be very dynamic, sometimes changing just as their orders are entered.
Another bit of jargon: the words ask and offer are the same thing. This is the side of the market where investors can buy stock. So when you hear: Where's the stock offered? Or what's the "ask" on the stock? They're both asking the same thing.
The size of the market can help you decide on the timing of your purchase or the price. For example, if good old XYZ is trading at $20, and the bid size for the stock is 200 and the offer size is 5, that means there are 20,000 shares bid for and only 500 for sale (when you see the amount of stock bid for or offered, just multiply it by 100 for the actual amount of stock. If you see 999, that means there are at least 100,000 shares). If you're looking to buy the stock, you might want to get your order in quickly because if the buyer of the 20,000 shares gets excited and starts to buy all the stock around, no matter what the price, it will push up the price. On the other side of the trade, if you are a seller, you may want to wait a little while because that kind of size to buy suggests that maybe the price will be moving up if the buyer doesn't have patience and wants that XYZ stock NOW.
Of course, the buyer may not move from the $20 price, or may find another stock that is more attractive and buy that one instead. So you can't know with certainty what will happen with the stock's price. But then, except for death and taxes, certainty just isn't part of life or investing.
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