Overlock seams are the traditional stitched seams you'll see on most everyday clothing, with visible thread loops on the inside. They're common on budget ski underwear because they're cheap to produce, but they come with real drawbacks on the mountain.
The main issue is chafing. Those raised seam edges can rub against your skin under a backpack, a hydration harness, or even just tight-fitting bibs and mid-layers. After a full day of skiing—especially when you're sweating—the friction adds up. Overlock seams also create visible lines under fitted layers and add bulk that can feel uncomfortable under tight gear.
If you're only doing casual lodge laps or occasional ski days, overlock seams might be fine for the price. But for regular riding, spend a little more on flatlock seams at minimum—they lay flat against your skin and layer much more comfortably. Your shoulders and hips will thank you on long lift rides.