A ferronière is a fancy word for a piece of jewelry that people used to wear long ago. It's like a special kind of necklace that only goes around your forehead, like a headband.
You know how sometimes people wear headbands to keep their hair out of their face? Well, a ferronière was kind of like that, but it was also very pretty and sparkly.
In the olden days, ladies used to wear ferronières to show how fancy and rich they were. They were often made of gold or silver and had pretty gems or pearls on them.
Nowadays, people don't really wear ferronières anymore, but you might see them in museums or fancy antique stores. And now you know what they are!