Thermal shields are used to safeguard the bridge of the Invincible from incoming torpedoes.
A thermal shield represents a specific type of deflector shield, and it was utilized by Admiral Trench.
Similar to a deflector shield, thermal shields had the capacity to repel both laser-based attacks and physical projectiles. They could even endure a direct impact from a barrage of torpedoes. While these thermal shields were active, launching torpedoes was impossible because they couldn't penetrate the shield's barrier. Furthermore, once deactivated, the shields required a complete recharge before they could be redeployed.
The Separatist officer, Admiral Trench, famously equipped several of his warships with thermal shields. His flagship, the Invincible, was among them.
These shields proved highly effective during the Separatist's blockade of Christophsis and the subsequent Battle of Christophsis. However, the shields' lengthy recharge period ultimately led to Trench's seeming demise. He had lowered the shields to fire upon the Galactic Republic's experimental stealth ship. General Anakin Skywalker, piloting the stealth ship, managed to turn the tracking torpedoes back toward the Invincible before its shields could reactivate.