+2 AR cloak
+2 AR robe
What is AR or what does it stand for and what does +2 do exactly?

Used to be the way your armor was calculated, It stood for Armor Rating(AR) there were no break downs of resists like we have now. It was an overall rating which by various formulas determined if you took any damage and how much damage was absorbed/deflected by your armor.

Where did you see this? on the uo website? if so its way out of date (as usual) If this is actually on a robe or cloak in the game then more than likely they have become bugged. And could be considered a rare. If you get a robe/cloak today it adds 2 to physical resist.