Used to describe objects or structures designed to withstand explosions from bombs specifically. A bombproof bunker can survive a direct hit.
Pro Tip 2/3
Security Emphasis
Implies a very high level of security, often more than typical protection. Officials demand bombproof security for the visiting dignitaries.
Pro Tip 3/3
Metaphorical Use
Sometimes used metaphorically to describe robustness, even if there's no risk of actual bombs. This case is practically bombproof, it can survive any drop.