The document discusses the central processing unit (CPU) and its stack organization. It describes the CPU as having a control unit, arithmetic logic unit (ALU), and registers that store intermediate data during instruction execution. A stack is a storage device that stores items such that the last item stored is first retrieved. In CPU stack organization, a stack pointer register contains the address of the top item in the stack. Items are added to the stack by incrementing the pointer and writing to the new address (push), and removed by reading the top address and decrementing the pointer (pop).