> Bid [500 x $64.39], [$64.42 x 100] Ask <
It means that someone currently offers to buy 500 shares at the price of $64.39.
It means that someone currently offers to sell 100 shares at the price of $64.42.
In practice, it means that you can buy 100 shares at the price of $64.42, then, after, it will move to the next price level.
=> You can see the "depth" of offers, it's called Level 2 book:
Ask offers:
$64.42 x 100
$64.43 x 200
$64.46 x 100
$65.00 x 600
$97.00 x 100
$999.99 x 100
=> If you try to buy 400 shares, it will cost you:
100 shares x $64.42 = $6 442
200 shares x $64.43 = $12 886
100 shares x $64.46 = $6,446
Divided by 400 shares.
It means ~$64.435 per share, though you saw $64.42 as price.
If you send very large volumes, you quickly eat these price levels.
In our example, if you try to buy 2000 shares, you will get only 1200 shares.
You will have paid an average price of $145.39 per share.
So, yes it may not go through because, it is possible that there are not enough people interested to sell their share (that there is not enough depth, or not at an acceptable price).
And also, the SEC has certain rules (uptick rules, etc) that makes the stock to be frozen in case the price varies too much.