The Bunker Busters upgrade was made available later in the conflict, following the vast deployment of GLA Tunnel Networks. Once purchased from the Airfield, the missiles from the Stealth fighter will release a mid-sized blast when it hits the ground. It is made to clear out buildings that have troops inside them, and also clear out tunnel networks. It can also damage or kill infantry in the range of the blast (more damage is inflicted in the center of the blast). Also, vehicles and other known armor near the missile's hit place will fly off (e.g. Quad cannon, Technical, etc.) but doesn't receive damage when it lands, unlike the Combat Cycle.
The blast has a 3 second delay from when the missile hits the ground to when it explodes. If the general has quick reflexes, he can empty the Tunnel Network or civilian building before the blast.
Bunker Busters were originally intended for the Aurora Bomber. The voice that plays when the upgrade is finished is the Aurora's voice, and the introduction says that the Bunker Buster is an upgrade for the Aurora. It was probably moved to the Stealth Fighter because the Aurora Bomb's sheer power completely destroys most bunkers, eliminating the need to bust them, and because it fits excellently with the Stealth Fighter.