Insulation weight is measured in grams and tells you how much warming material is packed into the pants. More grams means more warmth, but also more bulk. The range typically runs from 0g (uninsulated shell pants) up to 100g for extreme conditions.
For most resort skiers and riders, 40-60g hits the sweet spot for everyday winter conditions. If you run cold or ride in bitter climates, bump up to 60-80g. Go with 20-40g if you're riding milder days or tend to work up a sweat. Shell pants (0g) are great for active riders and backcountry tours where you prefer to control warmth with separate base layers.
The biggest mistake is over-insulating. If you're hiking, skiing aggressively, or riding park, too much insulation will leave you sweaty and uncomfortable. Also, your legs generally need less insulation than your torso—so don't just match your jacket's gram weight.