The wiring in this diagram is for adding a new light fixture to a switched receptacle, i.e. one that is hot only when a switch is on. These are commonly used to turn a table or floor lamp on and off from a wall switch.
This diagram illustrates wiring for a circuit breaker with a built-in ground fault circuit interrupter. This 20 amp ground fault interrupter breaker is a form of gfci that can be installed at the circuit source. This kind of circuit is used for dishwashers, whirlpool spas and other locations where water contact is likely.
Here a new receptacle is added to a light fixture circuit. The connections at the light fixture are spliced and extended to the new receptacle. With this arrangement the new receptacle will turn off and on with the light fixture.
The source neutral wire on the receptacle is removed and spliced to the white running to the switch, and to a pigtail back to the receptacle neutral. At the switch, the neutral wire is needed to power some dimmer switches and is now required in most switch boxes.