Like others have said, the stock market pairs up willing buyers with willing sellers. The stock exchange keeps track of all the people who want to sell, how many shares they want to sell, and the price they have set. The "ask" price is the lowest price at which anyone is currently offering to sell their shares. Similarly there's a list of buy orders with prices. The "bid" price is the highest price on that list right now. You're bidding a few cents lower than that, so you are not first in line to buy shares right now.
Suppose the list is like this:
$25.22 (1,000 shares)
$25.21 (500 shares)
$25.20 (1,000 shares - 250 for you, 750 for other buyers)
If someone puts in a market order to sell 2,500 shares, they'll sell 1,000 of them at $25.22 to the first group on the list, 500 at $25.21 to the second group on the list, and 1,000 at $25.20. You'll get your shares for $25.20. At this point, all the offers to buy for $25.20-25.22 have been exhausted, so the "bid" price you'll see will probably go down to $25.19.
Suppose the seller only wanted to get rid of 2,000 shares. The bidders at $25.22 and $25.21 will still have their orders filled, but half of the bids at $25.20 will remain unfilled. At this point the remaining bidders could wait for another seller to come around at their original bid, or increase their bid to $25.21 in hopes of attracting a new seller who wasn't willing to sell at $25.20.
For popular stocks and ETFs the "bid/ask spread" is usually only a few pennies, new bids get entered all the time, and lots of shares trade hands. You can set your limit price a few cents below the last sale price to try and get a better deal; this will often be successful because the share price will go up and down throughout the day, but there's always a risk that it never quite dips down to your bid. When this happens, you may find yourself paying more than you would have paid if you had just put in a market order at the time you first decided you wanted to purchase shares.
