Concrete under garage door threshold refers to the concrete layer poured beneath the garage door’s metal threshold, which seals the gap between the door and the concrete floor. This concrete provides a solid base for the threshold, preventing moisture and debris from entering the garage and ensuring a smooth transition between the floor and the door opening. It is essential to pour this concrete properly, using a concrete sealer to protect it from moisture and weathering, and installing a reinforcement mesh to enhance its strength and durability.