Each bomb contains a large amount of compressed Vril contained in a force field. When detonated, the Vril expands rapidly, causing extreme damage as it turns all matter it comes into contact with into more Vril, which damages more matter. These weapons can easily destroy large spaceships, and can conceivably destroy an entire planet.
Vril bombs were created by an insane Tuoco scientist, who tested the bomb on a rebellious colony moon. The moon was destroyed in a couple of days.