It would be nice to know the context (the screen or app it appears on) but it's probably an enhancement of the security settings to a "defense" level, which I guess would be the maximum. Is it an option that you turn on and off? Or is it a setting that sends you to another screen to choose more options?

"Defense" probably means military "Grade" means level or quality "Security" means things done to ensure safety (for a phone, probably things like encryption, firewalls, and password protection) So "defense-level security" probably means "military-grade safety measures." My guess is that Samsung is saying that the safety measures provided for the smartphone meet the standards set by the military for smartphones.