Tack has several meanings.
The tack is the direction of sailing so that the ship is sailing with the wind between the bow and the beam, named for the side of the ship which is exposed to the wind. Starboard tack, the wind lies off the starboard bow. The command would be 'Lay her on the Starboard tack'.
To tack or tacking means working, or beating, the ship or boat upwind by changing tack in succession.
A tack is a line attached to the clew of a sail to keep the foot (or base) taut or control the direction of the sail.
On a fore-and-aft sail, the tack is the lower forward corner of the sail, while the top corner is the head, the lower aft corner is the clew.