That is short for amoxicillin-clavulanic acid. One trade name for it is Augmentin. Basically, it is an anitbiotic (which is the amoxicillin in it), but it has an additive (clavulanic acid) that basically helps it become more effective against certain bacteria. It is prescribed for a wide variety of infectious conditions caused by bacteria (lower respiratory tract, otitis media, sinutitis, skin infections, urinary tract, etc.). It is typically prescribed against some bacteria that are able to produce an enzyme called beta lactamase, which makes them resistant to penicillin. Augmentin can supress that enzyme, thereby killing the bacteria. That is a very basic explanation, but I hope it helped.