Digital Registration on Entry
If you're entering Germany from another country and have visited a high-risk area in the last 10 days, then you're required to stay in quarantine for 10 days after entry.
You need to complete this Digital Registration on Entry so that the local health authority can check whether you're staying in quarantine. Your travel and contact details will be passed on to the local health authority at the place where you'll be staying so you can be contacted there.
Further information on current requirements for entry to Germany during the COVID-19 pandemic and who is not required to stay in quarantine is available on the website of the Federal Ministry of Health.