The term "Bunker" refers to the fuel used to operate ships. Bunkering is the action of supplying a ship with bunkers. The ship operator is responsible for paying these bunkers to the bunker supplier.
As fuel prices are volatile, the shipping lines charge a "Bunker Adjustment Factor (BAF)" fee to cover the fluctuations in global bunker costs.
This BAF is charged either as a separate or included in the freight charge, depending on the trade route.