What does this traffic sign indicate?

You answered correctly. This traffic sign marks the beginning of an autobahn (motorway) in Germany. When you see this sign, autobahn rules apply: only vehicles allowed on motorways may enter, there is generally no speed limit unless indicated otherwise, and stopping or parking is strictly prohibited except in emergencies. Recognizing this sign is important, as the driving rules and speed recommendations change immediately beyond this point. This does not indicate the end of the autobahn or a general clearway—only the start of the autobahn itself.