Pianos normally have pedals located under the keyboard that are operated by the performer's foot and effect the sound. There are typically two or three pedals. The left pedal is known as the soft pedal or muting pedal. The soft pedal is also known as una corda, Italian for "one string" because depressing the soft pedal shifts the entire keyboard to the right so the hammer only hits two of the three strings in the upper register of the piano and one string in the bass register. The effect is a softer sound since fewer strings are being struck.
The indication the depress the soft pedal is the term "una corda" (one string) under the bass staff, and the words "tre corda" (three strings) under the bass staff where the pedal is to be released.
See also muting pedal.