Plain English Meaning
The lowest weather conditions — how far you can see and how low the clouds are — that the rules allow you to land in when flying on instruments. If conditions are worse than these numbers, you cannot land from that approach.
Full Definition
The minimum visibility, runway visual range, and/or ceiling values prescribed by regulation for landing a civil aircraft while operating under Instrument Flight Rules. These minimums are published for each instrument approach procedure and must be met for the pilot to descend below the decision altitude or minimum descent altitude and continue to a landing.