Include a field into ITEMS configuration screen, in order to allow users describe what the items means. For instance, when creating ITEMS to monitor an UPS, like xupsBatCurrent, I would like to put "Battery Current as reported by the UPS metering. Current is positive when discharging, negative when recharging the battery."

And, this information should be presented in screens like latest data, using a mouse over (for example). So, if the user doesn't know what the item being monitored means, he will just put the mouse over the item, and a small ballon with the description will appear.

Another way is to put a little mark next to the item, like, [?], so the user could click it and learn what the item is monitoring.