Defense in depth, or DiD as people call it, is a way of protecting a system using layers. It is an approach that people use to protect their information.
Big companies, small companies, even individual users use this approach to protecting their information. When they put up a firewall, or any other protection for their system they put up multiple defenses. That is called DiD. Defense in depth is exactly how it sounds. There are multiple layers of security protecting information so when a hacker manages to hack through the first defense he has to hack through multiple more to get through your information.
