A gusseted crotch is a diamond-shaped fabric panel sewn into the crotch area that replaces the point where four seams typically meet. This design gives you more room to move and eliminates that uncomfortable seam intersection that can dig in or chafe during a long day on the mountain.
This feature is especially valuable for snowboarders, who constantly bend deep, sit in the snow, and move in ways that put stress on the crotch seam. Skiers benefit too—particularly if you're carving aggressively or hiking for turns—but the advantage is most noticeable for riders.
If you're shopping for performance base layers and spend full days on the slopes, a gusseted crotch is absolutely worth seeking out. Just don't assume all premium thermals include one—many don't, so it's worth checking the product details before you buy.