This is a very important sign. Whether with or without writing or coats of arms, in this case of Aargau, the yellow signs show you that you are on an official and marked path. It may not be the path you want, but at least it is a marked path and there are plenty of signs which makes it difficult to get lost. I managed though, I took two wrong turns today and lost a lot of time. But that's fine, I had loads of time, because it was only a four-hour walk and I wouldn't be let into the youth Hotel before five.