श्रीभगवानुवाच
इदं तु ते गुह्यतमं प्रवक्ष्याम्यनसूयवे ।
ज्ञानं विज्ञानसहितं यज्ज्ञात्वा मोक्ष्यसेऽशुभात् ॥
राजविद्या राजगुह्यं पवित्रमिदमुत्तमम् ।
प्रत्यक्षावगमं धर्म्यं सुसुखं कर्तुमव्ययम् ॥
śrī-bhagavān uvāca
idaṃ tu te guhyatamaṃ pravakṣyāmy-anasūyave |
jñānaṃ vijñāna sahitaṃ yat-jñātvā mokṣyase’śubhāt || 1 ||
rāja-vidyā rāja-guhyaṃ pavitram idam uttamam |
pratyakṣāvagamaṃ dharmyaṃ su-sukhaṃ kartum avyayam || 2 ||
In these Shlokas Bhagvan Shri Krishna agreed to impart to Arjuna one of the secrets knowledge which Bhagvan himself termed as "Raj Vidya" and "Raj guhyam". The word "Raj" here literal means of the top-level knowledge, regal science.
But the most important thing to be noted is in Shloka #1 and that is why Shri Krishna agreed to impart to Arjuna such secret knowledge? Since Arjuna has shown absolute faith in Shri Krishna and never had doubts, that makes him eligible for such knowledge which is of high importance.