What does “Steuer-ID” mean?
Steuer-IDEnglish: tax ID
In short
Your personal German tax identification number. It comes by post a few weeks after your first Anmeldung, and your employer needs it.
The Bundeszentralamt für Steuern posts your Steuer-ID automatically after your first Anmeldung. If it hasn’t arrived after a few weeks, you can request it again online.