While in port, ships need to have a range of services. Often ships need to be 'refueled' this is called bunkering. A boat called a bunker barge, rather like a floating petrol station, goes alongside the ship. The bunker barge has powerful pumps and loads fuel oil, called bunkers, into the ship's storage (bunker) tanks.
The transfer of fuel must be done properly and securely to prevent any oil spills.
There are other businesses that may visit the ship and take orders for food and other things the crew needs. These are delivered before the ship leaves.
The ship may also take on fresh water.