It is a monthly fee on FBA units stored a long time, generally past about 180 days. Amazon checks inventory age mid-month, charges the surcharge on top of your normal storage fee, and steps it up the longer stock sits. It works on a first-in, first-out basis, so it targets your oldest units first.
We order to real demand so inventory does not pile up past its shelf life, watch which units are approaching the age that triggers the surcharge, and clear the ones that need to go before they step up a tier. Day thresholds, tiers, and rates are set by Amazon and change periodically.