A thermal shield was a variant of the more common deflector shield used by Admiral Trench.
Like the deflector shield, thermal shields could deflect laser and projectile fire and could withstand a direct hit from a volley of torpedoes. Torpedoes could not be fired while thermal shields were up, as they would not be able to pass through the shield barrier. Once down, the shields could not be raised again until fully recharged.
Separatist Admiral Trench was known to have thermal shields installed on a number of his warships, including his flagship Invincible.
These shields were used to great effect during the Separatist blockade of Christophsis and the subsequent Battle of Christophsis. However, the recharge time of the shield resulted in the apparent death of Trench, after he lowered his shields to fire upon the Galactic Republic's prototype stealth ship. The stealth ship, piloted by General Anakin Skywalker, was able to redirect the tracking torpedoes back at the Invincible before its shields could recharge.