When you auction an item, you specify a minimum bid amount, a duration, and a reserve price. When the duration is over or at any time before, you can manually accept the best bid.
Note that OpenSea is currently imposing a mandatory 1 ETH or equivalent reserve price on English auctions, because of the recent substantial increase in gas prices. You can set a higher reserve price, but not a lower reserve price.
If no one placed a valid bid or if the highest bid is below the reserve price, you keep the item and the listing expires automatically without any attempt at automatic matching by OpenSea.
If you received at least one valid bid above your reserve price, the highest bid is above 1 ETH in value, gas prices aren't above 300 gwei, and the matching transaction costs less than 0.08 ETH, then OpenSea will cover the gas cost for you and automatically trade your item! If we automatically match the orders, you'll save some time and some gas money.