Left is the "soft" pedal...well, on grands, it works by shifting the hammers so they hit only 1 string rather than 2 or three. On uprights, it typically shifts the hammers slightly closer to the string so that there's less acceleration room. There are other ways of doing it, such as changing the angle of the hammers. The difference on an upright is often less substantial than the difference on a grand...
The right pedal is the damper pedal...holds all the felt off the strings so that the strings continue to sound after the key is released.
The middle pedal or sostenuto pedal is like the damper, but it only works on the key that are held while the pedal is depressed. It rarely is functional on uprights, and some pianos (even grands, some baby Yamahas come to mind) are not sold with them...