Breathability rating measures how much sweat vapor can pass through the jacket's fabric over 24 hours, expressed in g/m². The higher the number, the less clammy and damp you'll feel when you're working hard on the mountain.
For low-intensity resort riding in cold temps, 5,000–8,000 g/m² is usually sufficient. Most all-mountain riders will be well-served by 10,000–15,000 g/m². If you're hiking for fresh tracks, touring, or riding hard all day, step up to 15,000–20,000 g/m². Dedicated backcountry riders should seek 20,000+ g/m², where managing sweat is critical to staying warm and safe.
Don't confuse zippered vents with fabric breathability—pit zips help, but the material itself needs to let moisture escape even when fully sealed. Also, don't assume you can skip breathability just because you ride in cold weather; sweat freezes fast the moment you stop moving.