Ringmail is a new type of superior armor introduced in Mordenkainen's Magnificent Emporium. It requires a feat to be proficient with, and has a property - Durable II, as you noted - that separates it from other armor. The Durable property specifically gives a sort of one-use damage reduction each encounter. You can read up on it on page 8 of the aforementioned sourcebook.
Hope that helps!