A dealer to dealer transfer, sometimes also called a n FFL transfer, is when a firearm is shipped to a local dealer for processing and paperwork from a person or dealer from a different location usually out of state.
Firearms purchased on the internet will require an FFL transfer.
The Local dealer will forward a copy of their FFL to the selling party and receive the firearm for the end buyer. The buyer will complete the required paperwork and background check with the local dealer and upon completion of any required waiting period AND the clear background check deliver the firearms to the buyer/recipient.
Another type of transfer is a private party transfer, this is when two non-dealers sell a firearm between themselves and utilize a licensed dealer as an intermediary. The process is the same as an FFL transfer except the selling party is a non-dealer.