This is a brief sub-gloss. For a fuller reading of this verse, see Madhusūdana, Śaṅkara, or Rāmānuja above.

For the protection, the guarding, of the good, those who stand on the right path; for the destruction of the doers of evil, the workers of sin; and for the firm establishment of dharma, its right setting-up, I come to be, age after age, in every age.

The good, those whose conduct is the duty of the character described, foremost among the devotees of Vishnu, set out toward taking refuge in Me; and since My names, My deeds, and My own nature lie beyond the reach of speech and mind, they, gaining no happiness of self-sustenance, self-nourishing, and the rest apart from the sight of Me, count even a moment without Me as a thousand aeons and become slack in every limb. To deliver them by granting them the sight of My own form and My deeds, My converse and the rest; to destroy those opposed to them; and to re-establish the Vedic duty, which is the worship of Me and which had grown weak, by showing the true nature of the one to be worshipped; I come to be, age after age, in the form of a god, a man, and the rest. And there is not even the restriction of a particular age, the Krita, the Treta, and so on. This is the meaning.

There is no rule that the rescuing and the rest must be done by birth alone. Even so, He ranges as He wills, in play and by His own nature, as it is said, 'this is the very nature of God', 'play, free and entire, as in the world' (Brahma-sutra 2.1.33), 'behold His action, like that of a child at play', 'though of endless valour, He Himself dwelt apart from the city as if in fear of a foe', 'this One is full, and there is nothing for Him to gain, yet He sets all activities going; therefore the calmed sages who know the higher and the lower speak of Him as being amid contraries', and so on, in the supplementary hymns of the Rigveda.