Definition and background:
A strip of wood glued to the underside of the belly of a bowed stringed instrument that runs under the lower string. Its purpose is to help sustain the downward pressure exerted by the tension of the strings on the bridge.
For more information on the music term "Bass bar" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary