स एवायं मया तेऽद्य योगः प्रोक्तः पुरातनः।भक्तोऽसि मे सखा चेति रहस्यं ह्येतदुत्तमम्॥
sa evāyaṃ mayā te ’dya yogaḥ proktaḥ purātanaḥbhakto ’si me sakhā ceti rahasyaṃ hyetad uttamam
Word by word
The verse says
That same ancient practice is what I have told you today. You are devoted to me, and you are my friend. That is why - this is the best of secrets.
In plain words
Krishna gives the reason Arjuna is hearing this, and it is not the reason you would expect.
Not because Arjuna is worthy. Not because he asked well. Because bhakto 'si me sakhā ca - you are devoted to me, and you are my friend.
The qualification is a relationship.
Where you've felt this · in friendship
Your friend tells you the thing he has never told anyone. It is not because you are especially wise about it. It is because you are you, you were there at the time, and he has known you for twenty years.
Going deeper
This is the first time in the Gita that bhakta - devotee - appears, and it arrives almost casually, inside a reason rather than a teaching.
The order of the two words repays attention. Devotee first, and then sakhā - friend. Not one or the other. Both at once, which is a combination that ought to be unstable and is presented here as ordinary.
That pairing is the Gita's particular flavour. What is between these two is not the awe of a worshipper before a deity. They are cousins who have known each other for years, and one of them is driving the other's chariot.
Rahasyam uttamam - the highest secret. The word returns at the very end of the book, at 18.64, where Krishna calls what he is about to say the most secret thing of all.
So the chapter marks itself, at its opening, as the start of something more intimate than what came before. Chapters 1 to 3 were argument. From here the register changes, and you can hear it in how he is being addressed.