Insulation weight measures how much insulating material is in your jacket, listed in grams. More grams means more warmth but also more bulk. A 0g jacket is a shell with no insulation, while 150g+ is built for extreme cold.
For most resort skiers and riders, 80–100g is the sweet spot for everyday winter conditions. If you run hot or ride aggressively, 40–60g provides light warmth without causing you to overheat. Save 120–150g+ for frigid days or if you tend to run cold and spend a lot of time sitting on lifts.
Some jackets use body-mapped insulation and list two numbers, like 80g/60g—meaning more warmth through your core and less in the sleeves for better mobility. Also, remember that gram weight alone doesn't tell the whole story: different insulation materials have different warmth-to-weight ratios, so 60g of high-efficiency synthetic may feel warmer than 60g of a lower-grade fill.