This is where you enter your Ship From address, that will transfer into Seller Central when you go there to complete your shipment and purchase shipping labels.
You can make any of the shipping addresses primary, which will automatically fill that address into the batch Ship From section. To change the primary shipping address, click on the blue Edit button to the far right. In the Edit window, check the box at the bottom for Primary and save.
To add a new Ship From address, click on the green Add button in the upper right corner of the Shipping Information page. Fill out the details in the window that appears and click OK to save the information. You can also make this address primary as you enter it so that it shows up automatically in the ship from field on the batch creation window by default.