Pét Nat or 'Pétillant Naturel' means 'naturally sparkling' in french. Also called 'Methode Ancestral' or 'Methode Rurale', it indicates a wine that has finished it's alcoholic fermentation within the bottle. It has been around since before Champagne was discovered, which relies on a second fermenation in the bottle to gain its famous bubbles.
Expect a lightly fizzing, sometimes cloudy drink bursting of fruit flavour and a whole lot of fun.