A "furrowed brow" refers to the appearance of deep lines or wrinkles on the forehead, typically caused by the contraction of the muscles in that area. This expression often indicates concentration, worry, or confusion. When someone is deep in thought or facing a challenging situation, their brow may furrow as they focus their attention. The furrowing of the brow is a common human expression and can be observed in various situations, such as during problem-solving or when experiencing stress. It is a non-verbal cue that can help convey emotions and reactions without the need for words.