In plain words
One who knows Brahman and abides in Brahman, with steady buddhi and free from delusion, should neither exult on obtaining the pleasant nor be disturbed on meeting the unpleasant.
Word for word
One should not exult on obtaining the pleasant, nor be agitated on meeting the unpleasant: steady in buddhi, undeluded, the knower of Brahman established in Brahman.
न प्रहृष्येत् प्रियं प्राप्य नोद्विजेत् प्राप्य चाप्रियम्स्थिरबुद्धिरसम्मूढो ब्रह्मविद् ब्रह्मणि स्थितः
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
यस्मात् निर्दोषं समं ब्रह्म आत्मा, तस्मात् —
न प्रहृष्येत् प्रहर्षं न कुर्यात् प्रियम् इष्टं प्राप्य लब्ध्वा । न उद्विजेत् प्राप्य च अप्रियम् अनिष्टं लब्ध्वा । देहमात्रात्मदर्शिनां हि प्रियाप्रियप्राप्ती हर्षविषादौ कुर्वाते, न केवलात्मदर्शिनः, तस्य प्रियाप्रियप्राप्त्यसम्भवात् । किञ्च — ‘सर्वभूतेषु एकः समः निर्दोषः आत्मा’ इति स्थिरा निर्विचिकित्सा बुद्धिः यस्य सः स्थिरबुद्धिः असंमूढः संमोहवर्जितश्च स्यात् यथोक्तब्रह्मवित् ब्रह्मणि स्थितः, अकर्मकृत् सर्वकर्मसंन्यासी इत्यर्थः ॥ २० ॥
In plain words
Because Brahman, the Self, is without flaw and the same everywhere, therefore —
'Should not rejoice' means should not swell with elation upon getting what's pleasant, what he wanted, once he's gotten it. 'Should not be perturbed upon getting what's unpleasant,' what he didn't want, once he's got that too. For someone who takes the body alone to be the self, sure, joy and misery come from getting the pleasant and the unpleasant — but not for someone who sees the Self alone, because for him there's no such thing as 'getting' anything pleasant or unpleasant in the first place. And more than that: someone whose conviction is rock-steady, free of any doubt, that 'the Self in every being is one, equal, faultless' — that person has a firm intellect. Undeluded, free of confusion — such a person is a true knower of Brahman, established in Brahman, one who does nothing at all, having renounced every action. That's the meaning.
Close to the Sanskrit
Because faultless, the same, Brahman — the Self — [is], therefore —
'Should not rejoice' — should not produce elation — 'upon obtaining the pleasant,' [i.e.] the desired, having gained [it]. 'Should not be perturbed upon obtaining the unpleasant,' [i.e.] the undesired, having gained [it]. For those who regard the body alone as the self, joy and dejection arise from the obtaining of the pleasant and the unpleasant; not for one who sees the Self alone, since for him obtaining the pleasant or unpleasant is impossible. Moreover — 'one, equal, faultless is the Self in all beings' — he whose conviction, firm and free from doubt, is thus, is 'of firm intellect'; 'undeluded' — [i.e.] devoid of delusion — would be a knower of Brahman as just described, 'established in Brahman'; [i.e.] doer of non-action, the renunciant of all action. That is the meaning.