What makes seeing possible. Also of little weight — the same word for illumination and for ease.
Two distinct Old English words share this one spelling: leoht (brightness), from a root meaning to shine; and leoht (not heavy), from a separate, unrelated Proto-Indo-European root for lightness and nimbleness.
Two old words wear one spelling now — what makes seeing possible, and what carries little weight — and neither sense explains the other.