स एवायं मया तेऽद्य योगः प्रोक्तः पुरातनः। भक्तोऽसि मे सखा चेति रहस्यं ह्येतदुत्तमम् ॥

sa evāyaṁ mayā te ’dya yogaḥ proktaḥ purātanaḥ bhakto ’si me sakhā cheti rahasyaṁ hyetad uttamam

This same ancient yoga hath been to-day declared to thee by Me, for thou art My devotee and My friend; it is the supreme Secret.

In plain words: Krishna says he is now teaching this same old wisdom to Arjuna. He shares this highest secret because Arjuna loves him and is his friend.