You run the IT division for a large corporation. You want to create security for the system and set it up in multiple layers. That's graduated security. The defense against hackers involves a bunch of weapons.
Think of them as being a series of doors, each one with an increasingly elaborate lock. Also, as you enter each successive room (walk through the increasingly secure doors), fewer and fewer people get access to each area.
So the levels go from low security to moderate to high and, finally, to top secret. It increases by grades...i.e. graduated security.