Flatlock seams use a specialized stitching method that lays the seam flat against your skin instead of creating a raised ridge. On a ski balaclava, this means the seams won't dig into your forehead or temples under your helmet the way traditional bulky seams can.
This construction is the sweet spot for most skiers. It's more durable than seamless or bonded designs, so it holds up well to regular use and washing, but it's still low-profile enough to avoid those annoying pressure points during a full day on the mountain. You might feel the seams slightly if you press on them, but under a helmet they're barely noticeable.
If you ski with a helmet and want reliable comfort without paying a premium for seamless construction, flatlock seams are a great choice. They're the standard in quality performance balaclavas for good reason.