"Flat seams" refers to the smooth and level stitching found in clothing or fabric items. It means that the seams are sewn in a way that they do not create any bumps or ridges, providing a more comfortable and seamless appearance. Full definition
Made from wonderfully smooth soft stretch fabric, these mini briefs sit low on the waist and feature flat seams for an invisible finish under clothes. (kelkoo.co.uk)
Black 4 - way stretch leggings fit close to the body while flat seams increases mobility. (torrid.com)
It also has flat seams so nothing will rub or distract you while you're working out. (geekwrapped.com)