A rudder — the steering mechanism of a ship. James uses it as an analogy for the tongue: small but directing the entire course of one's life.
James writes: 'Ships are steered by a very small rudder wherever the pilot wants to go' (James 3:4). The tongue, like a rudder, is disproportionately powerful relative to its size. A ship's course is determined by this small instrument. Words steer lives. The direction of a person's existence is shaped by what they say.