Flatlock seams are created by overlapping two pieces of fabric and stitching them together so the seam lays flat against your skin rather than bulging up. This is the most common construction you'll find in quality ski underwear, and for most riders, it hits the sweet spot between comfort and price.
The big advantage is that flatlock seams won't rub or chafe the way traditional overlock seams can—especially important when you're wearing a backpack, carrying your skis on your shoulder, or sitting on cold chairlifts for hours. They also layer smoothly under mid-layers without creating awkward bumps or visible lines.
That said, you might still feel a slight ridge under very tight-fitting gear or if you have particularly sensitive skin. If that's a concern, seamless or bonded construction eliminates seams entirely but usually costs more. For the vast majority of skiers and snowboarders, flatlock seams are perfectly comfortable and a solid step up from budget options.