A measurement of how much wind has passed a given point in a period of time. A wind blowing at three miles per hour for an entire hour would give a wind run of three miles.
Cumulus calculates wind run by noting the average wind speed every minute (assuming the software remains running). Wind run is calculated by adding in a minute's worth of 'distance' corresponding to that speed. So the wind run for a particular period is effectively an indication of the average wind speed over that period. There are 1,440 minutes in a day. A wind speed measured in distance units divided by minutes, would give a wind run numerically equal to the speed each minute. A wind speed measured in distance units divided by hours, would give a wind run numerically equal to the speed when calculated for a period of an hour.
A wind run of 240 miles over the course of a day, for example, means that the average (of typically 1,440 measurements of) wind speed over the 24 hour day was 10 mph. Cumulus stores the wind run for each day in dayfile.txt.