What is the registration office and why do I have to register there?

It is a legal requirement that every citizen must register at their place of residence.

This must be done within 3 days after actually moving in (not after signing the tenancy agreement).

You can register your place of residence at any registration office, regardless of the district you live in. You will receive the registration form (Meldezettel) from your house management when you move in.

For more information can be found here.