Rilke was inspired to write this sonnet by a sculpture in the Louvre. You can see a picture of that torso at the ancientworlds.net link below.
The Greek god Apollo was associated with the sun, which is often used to symbolize him. He is also associated with healing and reform, which makes the last line more relevant. Also, one of the characteristics of marble is that it reflects light and has a sort of transluscence. So it can seem to glow.
Finally, to answer your question: The light Rilke talks about comes from two sources--the light of the sun god Apollo and the reflected light of the marble statue.