Defense in depth
Layering multiple independent controls so that one failure does not become a breach.
No single control holds. Email filtering misses messages, users click links, endpoint tooling misses techniques, and passwords leak. Layering means the attacker has to defeat several things in sequence rather than one.
The layers have to be genuinely independent to help. Three products that all rely on the same signature feed are one layer wearing three badges, and they will fail together.
Where this comes up
Defense in depth sits inside our cybersecurity and threats work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.