Shoshinsha mark

The Shoshinsha mark (初心者マーク) or wakaba mark (若葉マーク) is a green and yellow V-shaped symbol that new Japanese drivers must display on their cars for one year. (You must display this mark one in front of the car, and one on back of the car, after you take a standard driver's licence, for one year. This obligation is only for standard licence, not for motorcycle, large vehicle, special car, etc.)

Its official name is The Beginner Drivers' Sign (初心運転者標識, "shoshin untensha hyōshiki"). Conversely, the orange and yellow "fukushi mark" or "koreisha mark" denotes elderly drivers. Both marks are designed to warn other drivers that the marked driver is not very skilled, either due to inexperience or old age.

its become common practice for US drifters, and "jdm" fans to display these in their cars, as a tongue in cheek inside joke.