A directive to string musicians to perform the indicated passage with the bow at (more specifically over) the fingerboard of the instrument rather than in the usual position, which is between the fingerboard and the bridge.
See also [Eng.] at the fingerboard; [Fr.] sur la touche; [Ger.] am Griffbrett; [It.] sul tasto; [It.] sulla tastiera; [It.] flautando.
Video example: Sharon Polifrone, Assistant Concertmaster, Lyric Opera Orchestra, Chicago, Illinois