con sordino
With a mute; an instruction to play with a mute attached to the instrument.
In Depth
Con sordino means with mute and instructs the player to attach a mute to their instrument. For string instruments, the mute is a small clamp placed on the bridge that dampens the vibrations, producing a softer, more veiled tone. Brass mutes fit into the bell and come in many varieties.
The muted sound is distinctly different from simply playing quietly — it changes the timbre itself. Debussy, Ravel, and other Impressionist composers used muted strings extensively to create ethereal, distant textures. The instruction senza sordino tells the player to remove the mute.
The entire first movement of Beethoven's Moonlight Sonata is marked senza sordini (without mutes) — meaning the sustain pedal should be held throughout, creating a wash of sound.