Hypothalamus, Pituitary, and Testes. All hormones in our body work in whats called an axis. Basically an axis consists of two homrones that do the exact opposit. The body regulates these hormones by releasing one to reduce the other and vice versa to keep everything running smooth. Each axis also can effect other axis and so on. Since with AAS use, you are adding a hormone, you throw the axis off and when the body sense high levels of a hormone it halts or slows production, which is what we refere to as shutdown.
This if course is just a very basic explanation.