Pedal (music)
The pedal on the left makes the sound quieter.  It is called the "una corda" pedal (Italian for "one string").  This is because, in the early days of the piano, the una corda pedal made all the hammers move over so that a hammer would only hit one string for each note instead of three (most notes on a piano each have three strings to make a bigger sound).  On modern pianos the hammers actually hit two strings instead of just one.  They also tend to hit the string at a point where the hammer is softer, away from the grooves which get worn in the hammerhead.  On an upright piano the una corda pedal moves the hammers closer to the strings so that they do not hit them so hard.