The Profitability tab gives you a breakdown of what exactly goes into determining the estimated net profit amount shown on the Details screen.
You will see at the top the amount you have set up for your default Sales Tax and default Discount percentage from the Settings area.
To edit them for a product, tap directly on the number displayed. You will be taken to a new Update screen.
Tap on the number you would like to edit.
Enter the new amount and then tap Save Changes.
You will also see the estimated Amazon fees, the Cost/Unit you provided, the estimated shipping cost based on your entry on the Settings area, along with the estimated net profit total for both FBA and MFN.
These amounts will change automatically if you alter areas such as Cost/Unit, List Price, default Shipping cost, etc.