Denier measures the thickness of the threads in a jacket's outer fabric—higher numbers mean thicker, tougher material. A 20-denier jacket feels light and packable but can snag on branches or get sliced by a ski edge. A 100-denier jacket shrugs off abrasion but is noticeably heavier and bulkier.
For most resort riders, the 40–70 denier range hits the sweet spot—durable enough for daily use without feeling overbuilt. Backcountry tourers who count every ounce often go lighter (10–30 denier), accepting that they need to baby their gear a bit more. Freeriders charging through trees or scraping rocks should look at 80–150 denier, or jackets that use reinforced fabric in high-wear zones like shoulders and hem.
The key is matching durability to how rough you are on your gear. If you're regularly bushwhacking or dragging against surfaces, go higher. If you mostly cruise groomers, standard denier is plenty.