To play the rhythm of the song?
To play chords over a given structure ?
Like it would be Bob COMPING away during Rider while Jerry solos....
To back another musician while they are soloing?
I think its more a jazz term...
It's simply an abreviation for "accompanying", which as TJedi said,is of course to play over changes while someone takes a solo. It is definitely a jazz term and there's clearly a lot to be said about how one goes about doing this exactly.
I always try to enhance the soloist's playing somehow. Sometimes it's hard to do that and not interfere, stewp-on or detract from the solo. Of course, Bob did this in his own initimatable way.
Once in a great while, Bob would kind of hijack a solo from Jerry...not too often but occasionally.
Some of my favorite comping is from Brent on Jerry solos in Samson, Fire, Shakedown....just about everything for that matter.