A bass run is a sequence of bass notes that ascend or descend between two chords. The common type of bass run is connecting the root bass notes of two different chords. For example, in the following example a bass run is used to connect the chords C to F and back again. The first bass run uses the bass notes C - D - E and F to ascend from the C chord to the F chord. These notes are then reversed to descend from the F chord to the C chord.
The next example uses bass runs between the G, C and D7 chords and as the bass run is played the picking pattern is continued with the right hand.