A term originating among expatriates living in Thailand, a visa run is a quick, one-day trip across the border of a neighboring country and returning. This is typically done on or the day before the expiration of one's visa. Exiting the hosting country and instantly returning "resets the clock" on a pre-determined visa length to allow the visitor to extend their stay in the hosting country. Some nations have different laws controlling one's ability to perform consecutive visa runs, such as shortening the length of the subsequent visa, or limiting how many times within a given number of months one can perform a visa run.
My 60-day Thailand tourist visa expires tomorrow, so I'm taking a visa run into Cambodia in order to stay another 60 days.