यदा यदा हि धर्मस्य ग्लानिर्भवति भारत। अभ्युत्थानमधर्मस्य तदाऽऽत्मानं सृजाम्यहम् ॥

yadā yadā hi dharmasya glānir bhavati bhārata abhyutthānam adharmasya tadātmānaṁ sṛijāmyaham

Whenever there is decay of righteousness, O Bhârata, and there is exaltation of unrighteousness, then I Myself come forth;

In plain words: Krishna says that whenever goodness weakens and wrongdoing grows strong in the world, he comes into the world himself.