What does the middle pedal on the piano do?

Answer 1

The sostenuto pedal does only one thing when it is depressed: it will catch and hold any dampers that are already fully raised from the strings. The dampers are those things that lay across the strings and go up and down when you play a key. When the sostenuto pedal is properly used it will keep the dampers which have been raised, up until the sostenuto pedal has been released. Soundwise, what you get is one note or a bunch of notes that will be sustained for as long as the sostenuto pedal is engaged. What makes it so cool is that you can use the right pedal (damper pedal) as well as the left pedal (the una corde pedal) while you are also using the sostenuto pedal. The only trick is to not let the sostenuto pedal up until you really and truly want to release the sounds you have been sustaining.

Answer 2

it sustains the notes and makes them quieter