An ad can be inactive if it wasn't created correctly in the first place or if it's blocked. If this happens, the storage cost begins to accrue, but it's not available for sale because the ad isn't active.
Your performance is measured by the percentage of your FBA inventory units that cannot be purchased on Amazon at this time. This is known as your stranded inventory percentage
The other metric you should consider is the number of inactive units, which indicates the total number of units without an active offer, and the Fix ads button, which lists the SKU units without an active offer. This can be arranged using the Fix stranded inventory