In New Orleans, it's the little windshield sticker you're supposed to get every year after taking your car to a registered mechanic and having the breaks, headlights, tail lights, horn, and paperwork examined. If you're caught without a current one by a cop you pay a fine.
All other states that require them, and Louisianians outside of New Orleans call them "inspection stickers," but people in greater New Orleans call them "break tags."
My break tag is expired. I hope I don't pass a cop.