Defense in depth aims to avoid accidents by embracing a safety culture, but also accepts that mechanical (and human) failures are inevitable. Any possible problem - however unlucky - is then anticipated and factored into the design with multiple redundancies. The goal, therefore, is to provide depth to the safety systems; akin to the way Russian dolls have several layers before reaching the core doll. When one element fails, there is another, and another, and another that still functions.
Good definition of defense in depth.
