Chordal accompaniment, thats the stuff that guitarists and pioanist do when someone else is playing the melody or soloing. Its often not given much focus, but is the stuff that can make or break a tune. The Jazz term for this is comping short for accompaniment.
Lets start with a look at some guitar chord terminology.which may help with explaination
Early jazz - No Guitar Banjo
Most of the first Jazz Guitarists started out on Banjo