The cargo hold allows you to take stock of the goods in your hold and - when plundering another ship - take goods from the captured ship into the holds of your own ship.
Just below the date and gold information, you will find the information about the capacity in the hold of your ship.
The information about the goods is organized as follows. The first number is the amount of goods in your hold. Next comes the name of the trade good followed by a number in brackets, which is the number of goods available on the ship you've plundered (if you are plundering another ship, otherwise there is none). To the far right are two buttons that allow you to Take goods from the other ship or Dump goods from your cargo hold. Below the name of the good, you will find information about the goods in your hold, such as the average price you have paid to acquire the goods.
If you want to Take or Dump all goods of a kind (instead of transferring them one by one, press the appropriate button and hold it down (i.e., long press). This will take or dump all the goods of that type.