परित्राणाय साधूनां विनाशाय च दुष्कृताम्। धर्मसंस्थापनार्थाय संभवामि युगे युगे ॥

paritrāṇāya sādhūnāṁ vināśhāya cha duṣhkṛitām dharma-sansthāpanārthāya sambhavāmi yuge yuge

For the protection of the good, for the destruction of evil-doers, for the sake of firmly establishing righteousness, I am born from age to age.

In plain words: Krishna says he is born again and again, in age after age. He comes to protect good people, to destroy those who do evil, and to set goodness firmly in place again.