The utilities bill (Nebenkostenabrechnung or Betriebskostenabrechnung) is calculated every year. It shows how much hot water and electricity you really used.
You already pay the estimated utility costs (Nebenkosten) every month with the rent. If you used more than you paid for, you get an invoice. If you used less, you get money back.
After you move out of an apartment, the landlord waits until the Nebenkostenabrechnung to return your deposit (Kaution).