In any circuit or electronics devices the outer cover or framework of the device or the circuit is called the Chassis.
Grounding is the process of determining a Zero Reference level point and connecting all other devices to it for a common return path for current.
It is a common practice to mount electronic components to a conducting metal sheet or a non-conducting plastic board with printed wires ( Printed Circuit Board – PCB ) , When the chassis or PCB is used for building the circuit it is a common practice to regard the conducting body of the chassis or PCB as the common ground.
Chassis grounding makes the return of current easier and efficient , because chassis is a good conductor it provides a good path for the return of current from various components in a circuit. As the chassis is grounded so it is considered to be in Zero Reference Level and all voltage measurements in the circuit are done with respect to the chassis.
When using a Metal Chassis the body of chassis itself acts as the chassis ground and when using a PCB a rim of solder around the edge of the PCB acts as the ground.
The following figure shows how we can denote chassis ground in a circuit and how the chassis ground serves as a return path for the current: