Defense-in-depth is a security approach that uses several independent layers of protection, so that if one control fails or is bypassed, others still limit the damage. The idea comes from the recognition that no single control is perfect. A web server might combine timely patching, a web application firewall, network restrictions, least-privilege accounts, monitoring and tested backups.
The layers need to be genuinely independent to help. Two controls that fail for the same reason, or that depend on the same assumption, offer less protection than they appear to. A WAF in front of an unpatched application is one layer, not a replacement for the patch, and a protection that can be skipped, for example by reaching the server directly, leaves the layers behind it unguarded.
Having several layers does not prove that each one works. Controls can be misconfigured, set to log only, or silently disabled, and a layered design can still have gaps. Each layer should be checked on its own, rather than assumed to be effective because the others exist.
In practice, start with the controls that remove the vulnerability, such as vendor patches. Then add layers that make exploitation harder, restrict what a compromised process can reach, and make suspicious activity visible. Test them periodically and plan for the case where a layer fails, including how you would detect and recover from a compromise.