Plain English Meaning
Sensors next to the runway are reporting that a pilot landing there would be able to see 1,200 feet ahead along the runway. That is the visibility figure being used to decide whether the approach can be flown.
Full Definition
A reported Runway Visual Range value of 1,200 feet, meaning that a pilot in the touchdown zone of the runway can see runway markings, runway lights, or centerline lights for a horizontal distance of 1,200 feet down the runway. RVR is measured by transmissometers or forward-scatter sensors located near the runway and is reported in feet. A value of 1,200 feet is a common minimum visibility requirement for certain Category I ILS approaches and is roughly equivalent to a prevailing visibility of 1/4 statute mile.