As DPG has said its an RCD fused spur.
The RCD trips if it detects the types of faults which could be a person receiving an electric shock. This doesn't mean it only trips when a person receives a shock, other things will cause the same current flow that causes the RCD to trip such as dampness or damage to a cable.
The two buttons are test and reset, usually marked T and R.
The test button is used to test that the tripping mechanism is working, this causes the RCD to trip and switch off the power going out of the unit.
The reset button will reset the trip (switch the output back on) after it has operated either due to the test button or a real fault.
As per the label which is falling off these should be tested using the test button every 6 months (the label may say 3 months as this only changed to 6 months recently)