MON = Motor Octane Number.  The Motor Octane Number (MON) is a tougher test than RON and simulates higher engine speed and temperature.

Petrol is a complex mixture of hundreds different chemical compounds.  It does not have a fixed composition, but is graded according to its performance in a test engine, which results in an octane rating or number (RON or MON).
RON and MON tests are fuel octane rating tests that measure how good a petrol is at resisting engine knock.  The octane number is a value used to indicate the resistance of a motor fuel to premature detonation in the combustion chamber of an internal-combustion engine. In the long term, engine knock can lead to engine failure.