Most filters can emphasize the frequencies just before those that are reduced in volume. Resonance is the degree of emphasis that occurs. It makes the filter much more audible and gives it character - it would otherwise sound like just altering the equalisation - and is the foundation of the acid sound....
OB-Xa
An enhanced version of the OB-X, the OB-Xa also was "available in four-, six-, and
eight-voice configurations, offered 32 program memories, polyphonic portamento, and
polyphonic sample-and-hold", and in addition to that, it "offered the ability to split the
keyboard, or double up the voices so that two voices would sound for each key that was
depressed."