What is that?
A proof of identity is an official document. It shows who you are. You need it, for example, when dealing with authorities or submitting applications. If you are a citizen of an EU country, you can use the following documents, for example, as proof of identity:
Personalausweis (Identity Card)
If you are a citizen of a country outside the EU, you can use the following documents, for example, as proof of identity:
Aufenthaltstitel (Residence Permit)
Passersatzpapiere (substitute passport documents)
Where can I get it?
Where you can get proof of identity depends on the document itself.