无足轻重 is a four-character idiom meaning 'not worth mentioning; insignificant'. It often describes a person, role, or matter that has no real influence or importance. You can use it as a predicate (e.g., 这件事无足轻重) or as an attributive (e.g., 一个无足轻重的角色). The literal meaning is 'not sufficient to affect weight'—the image is that something is so light it doesn't tip the scales.