An anti-torpedo unit is a unit equipped with a weapon that can intercept enemy torpedoes. All destroyers are anti-torpedo units, and there are various other anti-torpedo units. They usually fire another small projectile at the incoming torpedo, killing it and keeping it from hitting the targeted ship. Anti-torpedoes usually are slow to fire so they can only intercept a small portion of a large salvo of incoming torpedoes.
For a complete list of anti-torpedo units, see the anti-torpedo units category.