To be illuminated by buddha ancestors is to endeavor in the thorough practice of zazen. Those who are ignorant mistakenly think that buddha light is like sunlight, moonlight, or the glowing of a jewel. But sunlight, moonlight, or the glowing of a jewel is merely a physical manifestation in the transmigration through the six paths, and cannot be compared with buddha light. Buddha light is to receive and listen to one phrase of teaching, to maintain and guard one dharma, and to transmit zazen person to person. Without being illuminated by such light, accepting and maintaining zazen is not possible.
From Beyond Thinking: A Guide to Zen Meditation by Zen Master Dogen, edited by Kaz Tanahashi, page 44