This is known as a shoshinsha mark, which are normally stuck on cars, it is basically a giant magnet. When I was in Japan I saw them everywhere, at first I though it was some fan club, or something to do with fishing. Because I encountered this symbol many times while playing ameba pigg. I also wanted to ask my host mother, but I didn’t know how to describe it in Japanese(^^;)
That was until PDSさん(Dante) was learning to drive. At he said the name of this item. They are mandatory to have on your car after you obtain your license for a period of 1 year, to alert drivers that you are new on the road. There is also another mark called a ‘Koreisha Mark’ for the elderly.
Shoshinsha can be used to mean beginner of anything, not just driving. For example it might be your first playing FFXIV online, so you follow this mark to a guide for beginners.
They also make for a cool case badge!!