Plain English Meaning
On final approach, the equipment near the runway is reporting that you can see about 2,400 feet down the runway from the touchdown end. For a standard ILS approach, that is usually the lowest visibility you are allowed to land in.
Full Definition
A Runway Visual Range value of 2,400 feet, meaning the horizontal distance a pilot in a moving aircraft on the runway can be expected to see down the runway from the approach end is 2,400 feet, as measured by transmissometer equipment near the touchdown zone. This value is a common minimum visibility requirement for a standard Category I ILS approach.
Context Anchor
Seen on instrument approach minimums, ILS approach tables, and for -visibility runway operations.