Square fill light ring light is a lighting tool, typically used in photography and videography, that provides even, diffused illumination, reducing shadows and creating a soft, flattering light on the subject. Unlike traditional ring lights, it has a square shape, which can create a unique catchlight in the subject's eyes. It's often used as a fill light to soften shadows created by a key light, ensuring details in darker areas are visible and the overall image is well-exposed.