The waterproof rating (measured in millimeters) tells you how much water pressure a jacket's fabric can resist before leaking. Think of it as how hard it can rain or snow before you start getting wet. For most resort riders, 10,000–15,000mm is the sweet spot—it handles moderate to heavy snowfall without being overkill.
If you mostly ride in dry, cold climates or on fair-weather days, 5,000–10,000mm will save you money and still keep you dry. But if you ride in wet coastal climates like the Pacific Northwest, or you're chasing deep powder days, look for 15,000–20,000mm. Go 20,000mm+ for backcountry use where staying dry is safety-critical.
One thing to watch: don't chase the highest number at the expense of breathability. If you're hiking or touring, a super-high waterproof rating can trap sweat inside. Also, remember that construction and seam sealing matter—a well-made 10,000mm jacket can outperform a poorly sealed 15,000mm one.