According to MSR, the term "waterproof" means that all external fabric of their tent has been coated with exceptional polyurethane coatings, and the seams are watertight out of the bag. The "mm" rating refers to millimeters and is used to represent a standardized measurement of how waterproof a coating is. For example, a 1500mm coating can withstand a 1500mm (5') column of water for over a minute before a single drop might appear through the fabric. This level of waterproofing is strong enough to prevent rain from leaking into the tent even in a hurricane-force storm.