The tax ID (Steueridentifikationsnummer) is a unique identification number all German residents have. This is the number the tax office (Finanzamt) uses to identify you. It has the format "12 345 678 901". You get that number after you register your address in Germany.
The tax ID is also called a Steuerliche Identifikationsnummer, Persönliche Identificationsnummer, Identifikationsnummer, Steuer-IdNr., IdNr or SteuerID. The tax ID is not the same as the tax number (Steuernummer).