Some pianos only have two pedals, but most have three. The right pedal is used most often, it sustains the notes. The left pedal is often used as well, it makes everything much softer. But the middle pedal is not used as often, which is a shame as it is really useful.
On some upright pianos the middle pedal makes everything extremely soft. This is designed to allow you to practice without annoying everyone else in the house! Sadly its not actually very useful. It puts a piece of felt in-between the hammers and the strings, and although this makes the piano very soft, it changes the touch of the piano enough to effect the way you play.
On a grand piano the middle pedal is called the sostenuto pedal. It is extremely useful. It is like having an additional hand. Basically, which ever notes you are holding down when you press this pedal will continue to sound undamped, but none of the other notes will be damped. You can use if to hold down notes or chords while you play other things with your hands.
This works differently to the sustain pedal on the right which just undamps all of the notes equally. The wonderful thing about the sostenuto pedal in the middle is you can hold notes down with it, but then play the other notes with a light detached touch
A confusing thing is that you press the sustain pedal before you play the notes you want held, but with the middle pedal, the sostenuto pedal, you have to play the notes you want held first, and then depress the pedal. If you have never used this pedal try finding ways to use it. Many pieces can’t be played accurately without it. I really like how it can free my hands from holding notes down allowing me to play more expressively and improve my piano playing.