It means the person driving down the road honked at you. How would we know why? Maybe they were warning you of something, maybe your car is on fire, maybe they are just a jerk.

It could mean that he recognises you , there could be an accident further up the road , you could have left something on your roof . In fact it could mean absolutely anything ,

It means that "Honk If You're Horny" bumper sticker is working.

You are either in the way or ugly usually.