A syn lara is a Trill musical instrument, similar to a piano, played by fingering keys which in turn cause tuned strings to be struck.
Joran Dax was an accomplished syn lara player, and was composing a new work on the instrument at the time of his death. (DS9 - The Lives of Dax short story: "Allegro Ouroboros in D Minor", DS9 - Mission Gamma novel: Cathedral)