The annual car tax

In the United States, you have to register your car every year. The same is true in Japan, though they make it easy for the U.S. military community by coming to the base for a few days in April every year. Last year was a bit of an exception (they did it in June), but this year it's back to normal, apart from the COVID-related distancing.
There's one fee for a regular small car; those get white plates. Vehicles with particulary small engines pay a lower fee; those yellow places.

The stickers note the year of the Japanese era. For example, this is 令和 (Reiwa), year 3.

To apply the sticker, you have to first peel the old one off, then remove the plastic film on the new sticker. Once it's applied to the inside of the windshield, removing it will destroy the sticker, so you have to be careful to do it right the first time.