The middle pedal on a grand piano is called the "sostenuto" pedal. The left-most pedal is the "una corda" or "soft" pedal, and the right-most pedal is the "damper" pedal. The sostenuto pedal on a grand piano keeps the dampers up for the keys that are depressed at the time the pedal is depressed, thus allowing the sound of these strings to continue after the keys are released. You can continue to play the rest of the keys, even using the damper pedal, independently of the action of the sostenuto. As long as the sostenuto is depressed, those notes will continue to sound. So the pedal has to be used correctly, or some notes you don't intend to continue sounding will anyway.

On upright pianos, however, the middle pedal, if one exists, is typically a practice pedal, mute pedal or celeste pedal, which places a piece of felt between the hammers and the strings. This is intended to be used for quiet practicing, and it is rarely if ever used for musical purposes. Some uprights have a sort of mock-sostenuto pedal, which sustains only the lower strings. This is of fairly limited use. In some older uprights, the middle pedal is merely a duplicate of the left (soft) pedal. As a true sostenuto pedal requires a fairly complex mechanism, only very few uprights have one.

However, since it is used very infrequently, it is rarely missed by most players. The sostenuto pedal didn't become popular until 1874, when Steinway introduced it. Obviously, earlier composers did not use it! Even Debussy (1862 - 1918) did not have a piano with a sostenuto pedal. Many modern concert pianists avoid the sostenuto pedal entirely.