A GFCI (Ground Fault Circuit Interruptor) or an RCD (Residual Current Device)
receptacle is designed to help prevent electrical shock to users of appliances plugged into them. The device constantly monitors the currents flowing in the hot and neutral conductors.
The currents flowing in the hot and neutral conductors should match almost exactly, to within a few milliamps. If the current flows do not match, the device disconnects itself within a few milliseconds, to stop the supply of electrical power to any receptacles it protects.
Because the device does not actually need to check for any ground current, it can be installed without a ground wire. (Unlike the earlier type of GFCI device, which only checked for the presence of some fault current flowing to ground and so needed to be connected to a properly installed ground wire.)
If there is no grounded conductor installed, a GFCI wired in this manner, and any receptacles protected by it, are each required to be clearly marked with a label saying:
"NO EQUIPMENT GROUND".
This method of changing two-wire ungrounded receptacles to three-wire grounding receptacles is allowed - even though they are not physically tied to the ground - only in a circuit which is protected by a correctly installed GFCI or RCD.
For more information about MCBs and related topics such as ELCBs, GFCIs and RCDs, see the answers to the Related Questions and the Related Link shown below this answer.
As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.
Before you do any work yourself,
on electrical circuits, equipment or appliances,
always use a test meter to ensure the circuit is, in fact, de-energized.
IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB
SAFELY AND COMPETENTLY
REFER THIS WORK TO QUALIFIED PROFESSIONALS.