"Head up" is a sailing command to steer the boat's bow closer toward the wind, tightening the sails. It reduces the angle between the boat's heading and the wind direction, acting as the opposite of "bearing away". It is frequently used for steering upwind, preparing for a tack, or reducing heel.

Key Details on Heading Up:
Synonyms: Include bear up, come up, go up, harden up, or luff up.

Action: Requires turning the helm (rudder) to move the bow toward the wind.

Sail Changes: As the boat turns into the wind, sails must usually be trimmed (hardened) in to maintain power.

Purpose: To sail closer to the wind (closer-hauled), change direction (tack), or respond to a change in wind direction.

Risks: Heading up too quickly or too far without managing sail trim can cause excessive heeling or cause the boat to stop (go "into irons").