The Subject Alternative Name, or SAN, is a certificate field that lists the hostnames and IP addresses a certificate is valid for. Modern clients check SAN entries when deciding whether a certificate matches the service they are connecting to, which makes it a core part of TLS trust.
SAN matters in cyber security because it can reveal more than identity. If administrators include internal hostnames, management interfaces, or network addresses, the certificate can expose infrastructure details that help an attacker map services and trust boundaries. Defenders also use SAN carefully to limit certificate scope, separate environments, and avoid accidental disclosure. Reviewing SAN contents is therefore part of secure certificate management, not just administration.