When you open a "Sell" order it means that you are betting on a price decrease and want to take advantage of it. This is also called a Short Order. A short order is created by taking out a loan denominated in the asset that you wish to short, with collateral denominated in USDC (or an other stable coin). The loaned asset is immediately converted to the stable currency. If the asset's price falls, the value of your debt also falls, meaning you need to pay back less money, giving you a profit. If the price rises, your debt increases, and you lose money.