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, and 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. A wind run of 240 miles over the course of a day, for example, means that the average (of up to 1,440 measurements of) wind speed over the day was 10 mph.