It's so only notes you choose will ring.
Left pedal: makes sound softer.
Right pedal: holds down all notes.
Middle pedal: SHOULD hold down only the notes you happen to be holding at the time. Other pianos use it as a pedal that makes sound really quiet so you can practice without disturbing. Still other pianos just make the sound echo a little bit. It doesn't even exist on many pianos, particularly European ones.
-a music major
Nobody's sure what the pedal to the far left does
It makes it softer and kinda muted-sounding
Any notes played before you push down the pedal will continue to ring, and notes played after you push the pedal down will be normal.
It does what the two people above me said on grand pianos. On a lot of upright pianos it acts as a bass sustain pedal.
Additionally, on some models of pianos, the middle pedal is a practice pedal that makes the piano incredibly quiet so as not to disturb others (not to be confused with the far right pedal).