For an MSR tent, waterproof means that all external fabric has been coated with our exceptional polyurethane coatings and the seams are watertight out of the bag. "mm" refers to millimeters and is paired with a number to represent a standardized measurement of the waterproof coating.
For instance, a 1500mm coating will withstand a 1500mm (5') column of water for more than one minute before a single drop might appear through the fabric. That's strong enough to prevent rain from leaking into a tent in a hurricane-force storm.