Just joined the forum and a camping carist living in France.
the larger French cities have problems with pollution at certain times of the year. they have introduced the stickers as a means of controlling vehicles that cannot be used when the pollution is high. the sticker is linked to the emissions rating of your vehicle; the higher the emissions, then the higher the sticker number.
when the city expects high level of atmospheric pollution, then they ban vehicles of a certain level being driven in the cities. as an example, in Paris when they last experienced high atmospheric pollution, they banned stickers number 5 (the highest emission emitting vehicles) from being driven inside the city.
They also fine vehicles NOT displaying a sticker!
This is only for some major cities at present 5Paris, Lyon for example) although others are considering introducing this (Lille is one). It is also only at times of high atmospheric pollution levels.
Hope this helps to understand?