When I say “walking chords”, obviously I don’t mean you should put a leash on your chord progression and take it out for a walk. Walking chords is bridging two or more chords with a single note run that utilizes lower pitch strings. We call it “walking” because, in effect, you are walking up or walking down in pitch one step at a time.
In country, folk, and blues music you’ll find opportunities to “walk” from one chord to the next. This works best for I, IV, V chords, but in theory, can be done between any two chords.