annabrahmā rasoviṣṇuḥ

This first part of this shloka states that the food (anna) is Brahma, the essence (rasa) is Vishnu, and the one who partakes of it (bhoktā) is Maheśvaraḥ (Shiva). This establishes the sacred trinity (Brahma, Vishnu, Shiva) in the act of eating - the food itself represents the creator Brahma, its taste or essence represents the sustainer Vishnu, and the person eating represents the transformer, Shiva. The second part teaches that one who eats with the sacred understanding described in the first half - where food is seen as Brahma, its essence as Vishnu, and the consumer as Maheśvaraḥ - becomes free from the various doshas (impurities) that can arise from eating.