In the port you can perform many actions, such as buying & selling goods, equipping a ship, or upgrading to a larger ship.
Use the navigation menu on the left side of the page to access the different options that the port provides.
All actions will be related to your currently active ship. When you buy a good, it will be added to your active ship, and the gold will be debited from it.
The current active ship is displayed below the name of the port. To switch to your active ship to another ship that you have at the current port, go to the shipyard and click the button "set to active ship" next to the desired ship.
There are currently 32 ports in the game.