FFL or Federal Firearms License is something that gun dealers have to have before they can sell or transfer a gun to you. Every gun dealer has one or more depending on the types of guns they sell. If you are buying a gun, you’ll be visiting an FFL holder and filling out some paperwork for a background check. Some states may also require additional paperwork for certain types of guns.
Many FFL holders are willing to handle the transfer of guns purchased online for a reasonable fee that covers their time and effort to deal with all the paperwork and other labor. For some, it is an important part of their business. Nobody can be in the business of selling guns unless they have an FFL, and all FFL holders have to perform mandated background checks before they can sell you a gun.