A registration certificate (Anmeldebestätigung or Meldebestätigung) proves that you live at a certain address. It looks like this.

You get an Anmeldebestätigung when you register your address (the Anmeldung). You need an Anmeldebestätigung to open a bank account, get a cellphone contract, etc.

The Anmeldebestätigung is sometimes called a Meldebescheinigung. Those are different documents. The Meldebescheinigung is a copy of the original Anmeldebestätigung. It proves that you still live at the same address. You only get an Anmeldebestätigung once, but you can request a new Meldebescheinigung anytime.

For more details, see this guide.