Items "on back" are how many of a menu item you the cook on a station have that are ordered in but are not yet fired.
Expediting chef: "order in: 2 steak M.R."
Grill cook: "2 steak M.R. How many do I have on back?"
Expediting chef: [looks at board] "With those 2, you now have 5 steaks on back."
Similar to "All Day". All day would include what's on back as well as what you are currently fired on. Eg: 'You have 5 steaks on back plus the one you are fired on right now, so that's 6 all day.