七の仏の nana no hotoke no
ふる言は furu kotoba
まなぶに六つの Manabu ni mutsu no
道に越えたり michi ni koetari
Primordial words are!
By studying them,
We go beyond the six realms.
"Seven Buddhas" refers to the Buddhas in the past, of which Shakyamuni is the seventh. Indian Buddhists thought there were Buddhas before Shakyamuni, and the idea of seven Buddhas in the past (Skt. saptatathagata) appears in Pali Nikaya and Chinese Agama, before Mahayana Buddhism.
"The primordial words" is a translation of "furu-kotoba" literally old or ancient words that, in this case, refers to the verse of admonitions commonly taught by the seven Buddhas. The most well known verse is by Kasyapa Buddha, the sixth of the past Buddhas: "Do not what is evil. Do what is good. Keep your mind pure. This is the teaching of Buddha." This verse also appears in the Dhammapada.
In the Dhammapada, there is another verse saying,
"Some people are born on this earth; those who do evil are reborn in hell; the righteous go to heaven; but those who are pure reach Nirvana."
The traditional understanding of these verses is that the first two lines on not doing evil and doing good are about transmigration within samsara based on causality, and the third line about keeping your mind pure is the teaching of going beyond samsaraand entering Nirvana.
Dōgen wrote a fascicle entitled Shobogenzo Shoaku Makusa (Not Doing Evil). In the very beginning of the fascicle, he quotes this verse and says,
"This [teaching], as the general precept of the ancestral school from the Seven Buddhas, has been authentically transmitted from former buddhas to later buddhas, and later buddhas have received its transmission from former buddhas. It is not only of the Seven Buddhas: it is the teaching of all buddhas."
He also wrote,
"This being so, when we study the supreme unsurpassable true awakening (anuttara-samyak-sambodhi), when we hear the teachings, do practice, and verify the result, it is profound, far reaching, and wondrous. We hear of this supreme awakening, sometimes following a teacher and sometimes following the sutras. At the beginning, it sounds, 'Do not do any evil'. If we don't hear 'do not do any evil', it is not the true Dharma of buddhas; it must be a suggestion of demons. We should know that, that which says, 'Do not do any evil' is the true dharma of buddhas."
In this passage, we see that Dōgen interprets the teachings in the verse as integration of the two sets of teachings: worldly dharma (Skt. laukika) causality based on good and evil actions, and the ultimate awakening beyond six realms (Skt. lokottara). Bodhisattvas go beyond samsara and yet, they don't escape from samsara, just as if a lotus flower blooms beyond the surface of muddy water and yet the root is still in muddy water.