It is intended to protect you from electric shock and your wiring and breaker panel from being shorted out . This way the single GFCI can trip or shut off if one of the receptacles further down the line are shorted out.
Electrical basics are easy to understand, but working on electrical projects can be hazardous.
You should always take the proper precautions turn off the circuit you are working on and use tools with insulated handles. Advanced - 15 to 20 minutes Make sure you turn off the circuit you will be working on. Locate the breaker or fuse and properly disable. Make sure that the amp rating of your new GFCI outlet is consistent with the amp rating of the wiring and the breaker or fuse. Make sure you connect the wiring to the correct terminals on your new GFCI outlet.