[xcasperx]

The spread (bid-ask spread) is the difference between the bid (what the seller is willing to accept to sell it) and the ask (what the buyer is willing to pay for it).

When you buy a stock, you're buying at the ask price, when you sell you're selling at the bid price. The difference is what the clearer keeps.

https://www.investopedia.com/terms/b/bid-askspread.asp

[pgwhalen]

This is mostly correct except for the last sentence, for any conventional definition of "clearer." You could replace "clearer" with "market maker" to be more correct, but it is important to note that the market maker isn't earning the spread every time they trade with you. They're only earning the spread in the aggregate, which is to say that they hope to both buy and sell with many people like you so that over time they are effectively earning the spread.