With a fixed auction, you set the lowest price in (Ether) you are willing to sell your NFT for and if a buyer wants it, can buy it at that price. Note that it is free to list an NFT for sale and to lower the price, however, will cost a transaction fee to cancel a listing and raise the price. Notice you can set a time period for how long you want to list it for under "schedule listing" as well as sell it as a bundle along with other NFTs. You can even reserve the listing for a specific buyer. Also notice the service and creator royalty fees at the bottom (circled in red). The service fee goes to OpenSea and the creator royalty is set by the creator of the NFT. In this case, 2.5% of your sale would go to OpenSea and 5.0% would go to the creator.