As others have posted, it means your room assignment is up to the sole discretion of the hotel.
If they aren't very busy, chances are you won't be put in that crummy room with the maid closet on one side, the ice machine on the other and a window that looks out upon a brick wall 2 feet away. But you certainly
could find yourself there. Especially if the hotel is booked solid during your visit.
If you have elite status with Starwood, I'd say your chances of getting the crummiest of rooms is greatly reduced, as often they oversell the cheapo room category and actually have lots of better rooms to play musical rooms with and distribute differently.
But anyone holding a Run Of House res should be prepared to put your best mooch forward at the check-in desk.
But don't be upset if you get just what you reserved: A room. Period.
Just a bit of minutia to add: It's usually referred to in the industry as "Run Of House" (without the "the") and you'll often see it displayed on confirmations and computer screens in it's abbreviated state of "ROH." Though sometimes it does have the "the" in there.