I would say that it’s a bumper sticker, an exclamation, to show that the person driving is fun-loving and country-loving.

Has similar vibes to YOLO (you only live once) as well.

It used to be slang for cocaine when I was in high school but slang changes over time and with location, so I don’t know if that’s still the case for this particular instance.

Aw, yeah
yeah yeah yeah yeah
yea yeah
yea yea, yea yea
yee yea
yee yee yee yea!
yee yee!

People are trying to tell you that you have a shit haircut, sorry :(

A word country people use when shits about to go down or they’re havin fun. And shows American pride