Definition and background:
soft instruments (literally, "low," but referring to volume, not pitch), suitable for the chamber. Includes vielles, rebecs and other bowed strings, lutes and other plucked strings, recorders, etc. Antonym: haut instruments.
For more information on the music term "Bas instruments" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary