In plain words
This is rāja-vidyā and rāja-guhya, the highest purifier, directly realizable, in accord with dharma, easy to practice, and imperishable.
Word for word
This [is] the king of knowledges, the king of secrets, the highest purifier, directly known, in accord with dharma, very easy to accomplish, imperishable.
राजविद्या राजगुह्यं पवित्रमिदमुत्तमम्प्रत्यक्षावगमं धर्म्यं सुसुखं कर्तुमव्ययम्
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
तच्च —
राजविद्या विद्यानां राजा, दीप्त्यतिशयवत्त्वात् ; दीप्यते हि इयम् अतिशयेन ब्रह्मविद्या सर्वविद्यानाम् । तथा राजगुह्यं गुह्यानां राजा । पवित्रं पावनं इदम् उत्तमं सर्वेषां पावनानां शुद्धिकारणं ब्रह्मज्ञानम् उत्कृष्टतमम् । अनेकजन्मसहस्रसञ्चितमपि धर्माधर्मादि समूलं कर्म क्षणमात्रादेव भस्मीकरोति इत्यतः किं तस्य पावनत्वं वक्तव्यम् । किञ्च — प्रत्यक्षावगमं प्रत्यक्षेण सुखादेरिव अवगमो यस्य तत् प्रत्यक्षावगमम् । अनेकगुणवतोऽपि धर्मविरुद्धत्वं दृष्टम् , न तथा आत्मज्ञानं धर्मविरोधि, किन्तु धर्म्यं धर्मादनपेतम् । एवमपि, स्याद्दुःखसम्पाद्यमित्यत आह — सुसुखं कर्तुम् , यथा रत्नविवेकविज्ञानम् । तत्र अल्पायासानामन्येषां कर्मणां सुखसम्पाद्यानाम् अल्पफलत्वं दुष्कराणां च महाफलत्वं दृष्टमिति, इदं तु सुखसम्पाद्यत्वात् फलक्षयात् व्येति इति प्राप्ते, आह — अव्ययम् इति । न अस्य फलतः कर्मवत् व्ययः अस्तीति अव्ययम् । अतः श्रद्धेयम् आत्मज्ञानम् ॥ २ ॥
In plain words
And that —
'Royal knowledge' — the king among all forms of knowledge, because it simply outshines everything else, the way this knowledge of Brahman blazes far beyond every other kind of knowing. In the same way, it is the 'royal secret' — the king among secrets. 'Purifying,' 'a purifier' — it is the supreme source of purity among everything that purifies: the knowledge of Brahman, unmatched. Even karma accumulated over thousands of lifetimes, merit and demerit alike, root and all, it burns to ash in a single instant. So really, what more needs to be said about its power to purify? And there is more: it is 'directly comprehensible' — known as immediately as your own pleasure is known, without anything standing between you and it. Even good qualities can sometimes clash with dharma, as we all know — but Self-knowledge never works against dharma; it is 'in harmony with dharma,' never departing from it. Even so, you might worry it must be hard to reach — so the text answers: it is 'very easy to accomplish,' the way recognizing a fine gem is easy once you know how. And there is a further worry it heads off: we normally see that easy, low-effort actions bring only small results, while hard ones bring great ones — so wouldn't something this easy have to burn itself out quickly? No — that is exactly why it says 'imperishable.' Unlike ordinary action, its fruit never diminishes. This is why Self-knowledge deserves your complete trust.
Close to the Sanskrit
And that —
'Rājavidyā': the king of knowledges, on account of possessing exceeding splendor; for this knowledge of Brahman shines exceedingly beyond all other knowledges. Likewise 'rājaguhyam': the king of secrets. 'Pavitram,' 'pāvanam': this is the supreme cause of purity among all purifiers, the knowledge of Brahman, most excellent. Even action rooted in merit and demerit, accumulated over many thousands of births, it reduces to ashes in a mere instant — hence what need be said of its purifying power? Moreover — 'pratyakṣāvagamam': that whose comprehension is, like that of pleasure and the like, gained directly — that is 'pratyakṣāvagamam.' Even something possessing many good qualities has been observed to conflict with dharma; not so is Self-knowledge opposed to dharma — rather, it is 'dharmya,' not departing from dharma. Even so, it might turn out to be difficult to accomplish — anticipating this, he says: 'very easy to accomplish,' as with the discernment of a gem. There, it has been observed that other actions requiring little effort, easily accomplished, yield little fruit, while difficult ones yield great fruit; but this, being easily accomplished, might therefore be thought to diminish through the exhaustion of its fruit — anticipating this too, he says: 'imperishable.' It has no diminution as regards its fruit, unlike ordinary action — hence 'imperishable.' Therefore Self-knowledge is to be trusted.