Definition and background:
A directive to string musicians to perform the indicated passage with the bow over the fingerboard of the instrument rather than in the usual position, which is between the fingerboard and the bridge.See also [Fr.] sur la touche; [Ger.] Am Griffbret; [It.] Sul tasto or Sulla tastiera.
For more information on the music term "Am griffbret" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary