The jpoc guide to priorite a droit in Germany.
I've given this page a French title but that is because most English speakers will have heard of the principle under that name. This rule is still used in parts of Germany and so you need to be aware that you may encounter it.
The idea is simple, if you are driving along and somebody is at a side turning on your side of the road (ie the right) then they have priority and you must give way to them.
I've only ever seen this in operation in built up areas and not always then. If the side street has a white line painted across it, then Priorite a droit does not apply but if there is no painted line, then the side street has priority over the main street.
This may seem like an anachronism but supporters of the idea point out that it actually increases safety. If cars can pop out of a side street at will, this causes the trafic in the main street to slow down in order to be able to stop if necessary.
Whichever is right, you should look out for this and check to see if the side streets near to you have priority.
You may also see this on roundabouts although those are anyway pretty rare in Germany.