The Inventory Performance Dashboard shows you a bar divided into four categories to help categorise the health of your inventory: Excellent (dark green), Good (light green), Sufficient (yellow) and Poor (red). If the status is Poor, alerts will be sent to notify you of the steps you can take to improve that status.
In terms of fees, in addition to being registered with Fulfilment by Amazon, you will also need to pay a monthly fee for inventory storage, which will differ depending on inventory volume, the type of product stored, whether the goods are hazardous or not, and storage dates. You can check all these details here
For extended storage periods
(where they exceed a certain number of days and depending on the type of item), Fulfilment by Amazon rates also vary.
If you still have any questions about how Fulfilment by Amazon works and all the things it can do for you as a seller, watch this explanatory video
that will clarify any doubts.