In plain words
Hear again My supreme word, the deepest secret of all. You are deeply dear to Me; therefore I shall tell you what is for your good.
Word for word
Hear again My supreme word, most secret of all; you are firmly beloved of Me, so therefore I shall tell what is good for you.
The commentators
Bhagavadgītābhāṣyam
भूयोऽपि मया उच्यमानं शृणु —
सर्वगुह्यतमं सर्वेभ्यः गुह्येभ्यः अत्यन्तगुह्यतमम् अत्यन्तरहस्यम् , उक्तमपि असकृत् भूयः पुनः शृणु मे मम परमं प्रकृष्टं वचः वाक्यम् । न भयात् नापि अर्थकारणाद्वा वक्ष्यामि ; किं तर्हि ? इष्टः प्रियः असि मे मम दृढम् अव्यभिचारेण इति कृत्वा ततः तेन कारणेन वक्ष्यामि कथयिष्यामि ते तव हितं परमं ज्ञानप्राप्तिसाधनम् , तद्धि सर्वहितानां हिततमम् ॥ ६४ ॥
In plain words
Listen once more to what I am saying —
This is the most secret thing of all — beyond every other secret, an utter mystery. Even though I've said it before, hear it again, once more: My own finest word. I'm not speaking out of fear, and not for any gain of my own — so why, then? Because you are truly dear to me, steadfastly, without wavering — that's the whole reason. So for that reason alone I will tell you what is good for you, the highest thing there is: the very means of reaching knowledge. Because that, of everything that could ever help you, helps the most.
Close to the Sanskrit
Again also, hear that which is being spoken by me —
Most secret of all, than all secret things, that is, utterly, extremely most secret, an utter mystery — even though [already] stated repeatedly, again, once more, listen to my — my — supreme, excellent word, statement. Not out of fear, nor for the sake of gain, will I speak; what then [is the reason]? 'You are dear, cherished to me — to me — firmly, through unwavering constancy' — thus taking this as the reason, therefore, for that reason, I will speak, I will declare to you — your — benefit, the supreme means of attaining knowledge; for that indeed is the most beneficial of all benefits.