It's known as the thin blue line - it denotes police.
A couple of years ago a police officer produced some of these to indicate to other officers that the car was owned by a relative of a police officer, such that if there ever was a problem, professional courtesy should be shown. However, these are now mass produced and many people slap them on their car thinking that it will save them from a speeding ticket.
I have a couple of police officer friends who have told me that they completely ignore these sort of stickers, police benevolent association tags and the like when deciding who to stop and what to do. A police officer's spouse is usually given some sort of mini badge that they keep in their wallet. If they get pulled over, they usually hand it over and that resolves the situation.