Text 64 

sarva-guhyatamam bhuyah
shrinu me paramam vacah
isto ’si me drdham iti
tato vaksyami te hitam

Translation

Because you are My very dear friend, I am speaking to you My supreme instruction, the most confidential knowledge of all. Hear this from Me, for it is for your benefit.

Commentary by Srila Prabhupada 

The Lord has given Arjuna knowledge that is confidential (knowledge of Brahman) and still more confidential (knowledge of the Supersoul within everyone’s heart), and now He is giving the most confidential part of knowledge: just surrender unto the Supreme Personality of Godhead. At the end of the Ninth Chapter He has said, man-manah: “Just always think of Me.” The same instruction is repeated here to stress the essence of the teachings of Bhagavad-gita. This essence is not understood by a common man, but by one who is actually very dear to Krishna, a pure devotee of Krishna. This is the most important instruction in all Vedic literature. What Krishna is saying in this connection is the most essential part of knowledge, and it should be carried out not only by Arjuna but by all living entities.

Commentary by Sri Visvanatha Cakravarti Thakur

Glancing at Arjuna, his very dear friend, standing silently as he began to review the very deep meaning of the Gita scripture, the Lord, his heart melting like butter due to his mercy, spoke.

"O my dear friend Arjuna, I will now speak the essence of the whole scripture in eight verses. Forget about the difficulties in having to review everything in order to understand it."

Again (bhuyah) hear the most secret knowledge. He says "again" because he said the same thing at the end of the ninth chapter:

man-mana bhava mad-bhakto mad-yaji mam namaskuru mam evaisyasi yuktvaivam atmanam mat-parayanah 
BG 9.34

Those were the highest words (paramam vacah), the most secret (guhyatamam), the essence of even the Gita, which is the essence of the meaning of all scriptures. There is no secret greater than this, anywhere, or from anywhere, in any way, at all.

He then gives the reason for speaking this. Because you are very (drdham) dear (istah), my friend, I speak for your benefit. Without having such friendship no one speaks any most confidential topic. Drdha matir (with fixed mind) instead of drdham iti is another reading of the text.