To nail with a tack (small nail with a flat head).
To sew/stich with a tack (loose seam used to temporarily fasten pieces of cloth).
To maneuver a sailing vessel so that its bow turns through the wind, i.e. the wind changes from one side of the vessel to the other.
To add something as an extra item.
to tack (something) onto (something)
Often paired with "up", to place the tack on a horse.