Defense in depth means multiple security layers. If one fails, others still protect you.
Physical. Locked doors, cameras, access badges.
Network. Firewalls, IDS/IPS, segmentation.
Host. Antivirus, host-based firewalls, application whitelisting.
Application. Input validation, authentication, encryption.
Data. Encryption at rest, access controls, data loss prevention.
User. Training, awareness, strong password policies.
Each layer catches what the previous missed. An attacker bypassing your firewall still faces host security.