Storage just means that they're launched and still in geosynchronous orbit but not actively used.
You can see the status of the Tracking and Data Relay Satellite (TDRS) Fleet on this NASA graphic:
Of the eleven TDRS satellites successfully launched into space, nine are still functioning and two (TDRS-1 and TDRS-4) are not. The two non-functional satellites were boosted into a super-sync orbit. Two of the nine functioning satellites (TDRS-3 and TDRS-5) are in storage.
From that you can determine two things. Stored doesn't mean retired, because the retired satellites are boosted in a super-sync orbit, but the stored satellites remain in geosynchronous orbit. Stored also doesn't mean non-functional, because the two stored satellites are counted with the nine functional satellites.
It's then not much of a leap to conclude that these two functional satellites in geosynchronous orbit are not currently being used, but are available to be taken out of "storage" and put into service if necessary.