Full form of CU is Control Unit.

The control unit is able to direct the operation of the system. Although, it does not perform any actual processing on the data, the control unit manages and coordinates the entire computer system.

It obtains instructions from the program stored in main memory, interprets the instructions and issue signals that cause other units of the system to execute them.
Actual line indicates the flow of instructions and data while dotted line indicates that control process has done by Control Unit.