In plain words
One whose mind is undisturbed in sorrow, free from craving in pleasure, and from whom attachment, fear, and anger have departed is called a sage of steady understanding.
Word for word
One whose mind is unshaken in sorrows, who is free of craving in pleasures, from whom attachment, fear and anger have gone, is called a sage of steady understanding.
The commentators
Bhagavadgītābhāṣyam
किञ्च —
दुःखेषु आध्यात्मिकादिषु प्राप्तेषु न उद्विग्नं न प्रक्षुभितं दुःखप्राप्तौ मनो यस्य सोऽयम् अनुद्विग्नमनाः । तथा सुखेषु प्राप्तेषु विगता स्पृहा तृष्णा यस्य, न अग्निरिव इन्धनाद्याधाने सुखान्यनु विवर्धते स विगतस्पृहः । वीतरागभयक्रोधः रागश्च भयं च क्रोधश्च वीता विगता यस्मात् स वीतरागभयक्रोधः । स्थितधीः स्थितप्रज्ञो मुनिः संन्यासी तदा उच्यते ॥ ५६ ॥
In plain words
And further —
'In sorrows' — the threefold sorrows life brings — when they come, someone 'not agitated,' not shaken, is what's meant by 'one whose mind stays unagitated in sorrow.' The same holds for pleasure: when pleasures arrive, someone whose longing, whose craving, has simply left him — pleasure does not, in him, blaze up higher and higher the way a fire grows when you keep feeding it fuel — that person is 'one whose longing has departed.' And 'one from whom attachment, fear, and anger have departed' means exactly that: attachment, fear, and anger have all left him. Such a person — of steady mind, of steady wisdom, a sage, a renunciate — is the one this word is naming.
Close to the Sanskrit
And what more —
'In sorrows' — the threefold sorrows — 'having occurred,' [one whose] mind is 'not agitated,' not perturbed, when sorrow occurs: he is 'one whose mind is unagitated.' Likewise, 'in pleasures,' when they occur, longing, craving, has departed from whom — [pleasure] does not, like a fire fed with fuel, grow with each pleasure received — he is 'one whose longing has departed.' 'One from whom attachment, fear, and anger have departed': attachment, and fear, and anger have gone, have departed, from whom — he is 'one free of attachment, fear, and anger.' One of steady mind, one of steady wisdom, a sage, a renunciate — he is then so called. [End of commentary on verse 56.]
Gītā Vāhini
సుఖములు కానీ, దుఃఖములు కానీ మూడు విధములు. ఒకటి ఆధ్యాత్మికము, రెండవది ఆధిభౌతికము, మూడవది ఆధిదైవికము. సుఖఃదుఃఖములు పుణ్యపాపముల సంబంధములు. పాపములను చేయవద్దని చెప్పుటకే దుఃఖములు వచ్చుచున్నవనియు, పుణ్యములు చెయ్యమని చెప్పుటకే సుఖములు వచ్చుచున్నవనియు తలంచవలెను. కానీ స్థితప్రజ్ఞుని, ఏ దుఃఖమూ బాధించదు, ఏ సుఖమూ ఆకర్షించదు. కష్టములకు కలతపడదు; సుఖములకు లొట్టలు కొట్టదు. ఆత్మ విషయము తెలియని వారే ఈ రెంటికీ పొంగి కృంగేది.
In plain words
Pleasures and sorrows alike are of three kinds: the first ādhyātmika, the second ādhibhautika, the third ādhidaivika. Pleasure and sorrow are bound up with merit and sin. You should think that sorrows come only to tell you not to commit sins, and pleasures come only to tell you to do meritorious deeds. But no sorrow troubles the sthitaprajña, and no pleasure draws him. He is not shaken by hardships; he does not smack his lips over pleasures. It is only those who do not know about the Ātma who swell up and sink down at these two.