Question: What does U.C. mean?
Albert's reply: U.C. stands for una corda and it refers to a piano's leftmost of the three piano pedals. It is also called the soft pedal because it is primarily used to soften the sound.
On a grand piano, the left pedal shifts the keyboard slightly to the right, which results in the hammers striking one less string. Most of the keys on a piano have three strings, with the lowest notes having only one string.
The name una corda literally means "one string" because when the pedal was first introduced (by the piano's inventor Bartolomeo Cristofori, at which time it was a manually operated stop) it shifted the action so that the hammers struck one string rather than two.
On an upright piano, the una corda pedal does not shift the action. Instead, it moves the hammers closer to the strings, creating two differences compared to a grand piano: First, it changes the sound by giving the hammers less distance to travel, resulting in a softer sound, but not one in which the color is changed. Second, it changes the feel of the action itself in an undesirable way.
It's important to note that the una corda pedal is not intended to be a crutch for pianists to play softer. Instead, its primary purpose is to change the color of the sound. Make sure you can play as soft as possible without the aid of the soft pedal—then experiment with pedal effects!