Bhashya
अप्सु ज्योतिः इत्यब्ज्योतिषोरन्नान्नादगुणत्वेनोपासकस्य अन्नस्य बहुकरणं व्रतम् ॥
In plain words
Understanding that light resides within the waters, and meditating on the dual principles of water and light as food and consumer, the seeker's sacred vow is to make food abundant.
Close to the Sanskrit
Light is in the waters, thus of water and light in the quality of food and consumer, the making abundant of food of the meditator is the vow.