A moving bass line is when the bass notes played in a chord progression move up or down a scale or pattern. In order to achieve this effect you need to be selective with the bass notes you choose to pick within each chord.
The following chord progression consists of the chords C, Em, Am and G. Normally the bass motes you would select for each chord would be C, E, A and G respectively. However if for the Em chord you select the 5th string bass note (B) the four main bass notes you will be playing are C, B, A and G, in that order. You now have a moving bass line that descends from the note C, to B, to A and then to G. When a different bass note is selected instead of the usual root bass note the chord name is altered accordingly. In the following exercise the B bass note is selected as the first bass note for the Em chord. This bass note will now become the dominating bass note. The chord name changes from Em to Em/B meaning an Em chord with a B bass note. Play the following progression listening carefully for the effect of the descending moving bass line.
The next example uses a moving bass line that ascends from the low E note to the D# note on the first fret, fourth string.