A Ground Fault Breaker/Receptacle is a device that shuts off an electric power circuit when it detects that current is flowing along an unintended path, such as through water or a person. It is used to reduce the risk of electric shock, which can cause the heart to stop or cause burns. They can also prevent some fires, like when a live wire touches a metal conduit.
A GFCI works by measuring the current leaving one side of a power source (“hot”), and comparing it to current returning on the other (“neutral”). If they are not equal, then some of the current must be leaking in an unwanted way, and the GFCI shuts the power off. After the problem is fixed, the device must be reset manually by pushing the reset button.
There is also a test button next to the reset button, which shows that the GFCI works properly. GFCI outlets should be tested at least once a month.
According to Canadian Electrical Code, any receptacle near water source (kitchen sink, wash sink, shower, tub etc) and outside receptacle need to be protected by GFCI to reduce the chance of harming people and causing property damages.