Maybe you need this magical number to apply for unemployment benefits or something else.
This article will explain things in plain English.
Rentenversicherungsnummer is a unique identifier to associate a person with their payments into the German retirement system. source
The word Sozialversicherungsnummer is just another name for it, but it is different than your tax number (Steuernummer.)
There are multiple places you can find it, this section will show you.
It is often abbreviated as SV-Nummer, SV-Nr, RV-Nummer, or RV-Nr.
You payroll documents may be called something like Gehaltsabrechnung or Entgeltabrechnung.
This document is from the pension office.
In order to get the document, you already need to know your number, but it can be helpful to confirm it.