Fortniet Blog delivers a short introduction what is Fortinet’s view on layered security:
Layered security combines multiple security measures to make sure that you don’t have all your security eggs in one basket. This protects you from different sources and different types of threats.
There are a few different models out there for layered security. The one we’ll be using has seven distinct layers. Below you’ll find a brief description of each layer, followed by a list of the relevant Fortinet features.
A good way to get a very short introduction to this topic.