A CS2 skin spread works the same way as a spread on a stock or a used car. Buyers post the most they will pay, sellers post the least they will take, and the space between the two numbers is the spread. If you want to sell instantly you take the buyer's price, and if you want to buy instantly you pay the seller's price. Either way, you give up the spread.
Why does the spread matter when trading skins?
The spread is the cost of speed. A patient seller lists at the ask and waits for a buyer to come to them. An impatient seller hits the bid and gets paid now, but for less. On the Steam Community Market you can see this directly: the buy order column shows bids, the listing column shows asks, and the difference is the spread.
Popular skins like an AK-47 | Redline have tight spreads because hundreds of people trade them every day. Rare items like a Karambit | Doppler with a specific phase and low float have wide spreads because only a few buyers exist at any moment. Wide spreads make rare skins expensive to flip. You can be right about the value and still lose money to the spread if you sell too fast.
How do you calculate the spread on a skin?
Take the lowest listing price and subtract the highest buy order. Divide that by the listing price to get a percentage. A 10 dollar skin with a 9 dollar top buy order has a 1 dollar spread, or 10 percent. Add the marketplace fee on top, and you can see why quick flips on cheap skins rarely pay. The trade profit calculator does this math for you.
Spreads also differ between sites. A skin can have a tight spread on Steam and a wide one on a cash marketplace, or the other way round. Traders who understand spreads shop for the market where the gap is smallest before they buy or sell.