A listing may become stranded if it was not created properly in first place or gets suppressed. If this happens, it starts collecting storage fees but is not available to be sold because the listing is not active.
Your performance is measured by the percentage of your FBA inventory units that can't be bought on Amazon right now. This is called the "stranded inventory percentage
The other metric you need to be aware of is the quantity of stranded units, which indicates the total number of units without an active offer, and the Fix Listings button, which lists the SKUs without an active offer. This can be sorted with the Fix Stranded Inventory option.