A bonnet is like a hat that is typically worn on a woman's head. It covers the top of the head and has a brim that goes all the way around to cover the forehead and the sides of the face. It is often tied under the chin with strings.
Picture a big, floppy hat that a lady might wear to the beach to keep the sun off of her face. Now imagine taking away the floppy part and making it smaller so it only covers the top of her head. That's kind of what a bonnet looks like.
Bonnets were very popular a long time ago, when ladies were expected to cover their heads when they went outside. They were often made of soft fabrics like cotton or silk, and decorated with lace, ribbons, or flowers. Nowadays, bonnets are mostly worn as a fancy accessory for special occasions, like weddings or Easter.