If an object is beaming, it's glowing with light, but if a person is beaming, they're more likely to be smiling brightly.
Your car's beaming headlights or the beaming eyes of your cat, glowing in the dark, are bright and radiant. That's where the other meaning of beaming, cheerful and bright, comes from. A beaming smile glows and shines with true joy. Beaming comes from the verb beam, "emit rays of light."