Knee-length means long enough, or tall enough, to reach the knees. We say it about clothes, and sometimes about boots or socks: a knee-length skirt, knee-length shorts, a knee-length coat. It tells you where the thing stops on the body — at the knee, not at the ankle and not at the hip. Though actually, “reach the knees” is a little loose: a knee-length skirt usually sits at or just below the knee, and fashion people argue about the exact line. We do not say “a knee-length child”; we save it for things that hang down. For grass, knee-high is more common than knee-length. It is a plain, useful describing word.