"Radiance" is a bright, warm light or glow that seems to spread out from something. You can use it for real light, like the radiance of the sun or a lamp. But people also use it for a person: if someone has "radiance," it means they look glowing with health, happiness, or beauty - their face seems to shine with good feeling. It's a warm, positive, slightly poetic word, often used to give someone a real compliment.