“Ocelli” is a strange word. It sounds like an instrument in an orchestra, but it's less important to think about what ocelli sound like than what ocelli see. An ocellus (from the Latin meaning “little eye”) is a primitive light sensor, and bees have three of these on the tops of their heads. This is, of course, in addition to their two big compound eyes that cover a lot of their face, like sunglasses on a fat Southern sheriff in a 1970s movie. Compound eyes are great for bees in that their many facets provide the widest possible range of vision in all directions. You'd think they'd be enough, but they aren't.
On the face of it, the ocelli don't make a lot of sense. They don't focus and are only good for registering gradations of light, like a photoelectric cell or a very foggy window. Simply registering light turns out to be a good thing though for keeping track of where the sun is. This is important for two reasons. First, it keeps the bee attuned to which direction is up, which isn't always clear in the dizzying world of flying. Secondly, the bee can navigate with the ocelli alone, saving their big compound eyes for tracking danger, obstacles, landmarks, and flowers.