When a person is focused on something, they're paying attention to it. When a camera lens or your eyes are focused, they've made the adjustments needed to see clearly. When a beam of light is focused on a thing, it's shining on that thing.
The adjective focused derives from the verb focus, which means to fix on a central point. When you are focused on school, all your attention is fixed on one thing: school. When your telescope is focused on Jupiter, you have aligned the lenses with where Jupiter is in the sky. And when your flashlight is focused on the base of your chin, the shadows cast on the rest of your face make you look really creepy.