What a GFCI Outlet Does
A ground fault circuit interrupter, or GFCI, compares the current flowing out on a circuit with the current coming back. If some of it is leaking along another path, the device switches off to help prevent a serious shock. GFCI receptacles have test and reset buttons on the face, which is how most people recognize them.
A GFCI receptacle protects whatever is plugged into it and, depending on how it is wired, other outlets further along the same circuit. That is why one GFCI sometimes controls outlets in another part of the room or even another room.
GFCI protection is not the same as equipment grounding. A ground wire gives fault current a safe path, while a GFCI watches for leakage and cuts the power. A GFCI can add shock protection on an older circuit without a ground wire, but it does not create a ground connection or correct grounding problems.
Pressing the test button as the manufacturer directs helps confirm the device still trips. If it does not, the GFCI should be checked.