The tax number (Steuernummer) is a unique number that identifies businesses and freelancers. It’s a 10 or 11 digit number, and has the format “12/345/67890”. It’s sometimes written in the unified federal format as “3012034567890”, where the first two digits are the number of your Bundesland.1
You obtain this number when you submit the Fragebogen zur steuerlichen Erfassung to the tax office (Finanzamt). The Steuernummer is not the same as a tax ID (Steueridentifikationsnummer) or VAT number (Umsatzsteuernummer).
Your Steuernummer is not private. It should appear on your invoices, and in your Impressum. If you move to a different area, your Steuernummer will change.