The control unit is the part of a computer that controls the computer's operation. Basically, each part of the computer requires control signals to arrive at particular times for each instruction of the computer's software. The control unit provides those control signals.
At one time control units were ad-hoc logic, and they were difficult to design. Now they are designed as a microprogram that is stored in a control store. Words of the microprogram are selected by a sequencer and the bits from those words directly control the different parts of the computer, including the registers, arithmetic and logic unit, instruction register, bus, input/output and computer storage.
In modern computers, each of these subsystems may have its own subsidiary controller, but the control unit is the top dog that controls the computer overall.