Exactly what it sounds - a face brush is specifically designed to be used on the horse's head and face.
A face brush is really just a very small body brush that fits more or less in the palm of your hand. Using a larger brush on a horse's head increases the risk that you'll accidentally poke a horse in the eye. I've also met horses that would not tolerate a larger brush near and around their head.
Not everyone considers a special face brush to be necessary, however. But if you see a very small and thin brush in a grooming kit, that's what it is.