Ring 0 is the highest privilege level in Windows and other ring-based operating systems. Code running there executes in kernel mode, where it can directly control memory, processes, devices, files, and registry state. Because it sits at the core of the system, Ring 0 code can bypass many of the restrictions that apply to normal applications.
This matters in cyber security because attackers who reach Ring 0 can disable defenses, hide malicious activity, or tamper with security settings before endpoint tools fully load. Legitimate drivers can also be abused here: a signed boot-time driver may have broad enough access to act as a defense-evasion primitive if its interface is reachable by an attacker. For defenders, Ring 0 is a high-risk trust zone, so monitoring driver loading, limiting privileged access, and validating kernel components are critical.