Middle C is the musical note C that is in the middle of the piano keyboard. It is not perfectly in the middle of the keyboard, but very nearly. When you compare all the Cs in the piano, it is the one nearest to the middle.
When writing Middle C in music notation, it is below the stave when using the treble clef and just above the stave when using the bass clef. This is shown in the first and last notes of the musical example below. The Middle C sits on a ledger line (an extension of the stave written just for one note).
On the piano the Middle C key is C4.
When starting to learn the piano, most books make their easiest pieces start on Middle C. This is because the right hand can then play tunes using 5 notes (C to G) using white notes only (no sharps or flats). It is also easy to recognize Middle C in notation. It is also a help to find the correct place to sit at the piano (Middle C is opposite the player's "middle" (tummy)). One problem can be that young children learning the piano may think that all pieces start on Middle C. Pieces can start anywhere on the keyboard.
Middle C is the lowest note of the flute. It is a fairly low note for a child or woman to sing, but a fairly high note for a man to sing.
Middle C vibrates at 262 Hz.