In plain words
The collected one, relinquishing the fruit of action, attains the peace born of steadfastness; the uncollected, driven by desire and attached to the fruit, becomes bound.
Word for word
The collected one, having relinquished the fruit of action, attains the peace born of steadfastness; the uncollected, attached to the fruit by the working of desire, is bound.
The commentators
Bhagavadgītābhāṣyam
यस्माच्च —
युक्तः ‘ईश्वराय कर्माणि करोमि न मम फलाय’ इत्येवं समाहितः सन् कर्मफलं त्यक्त्वा परित्यज्य शान्तिं मोक्षाख्याम् आप्नोति नैष्ठिकीं निष्ठायां भवां सत्त्वशुद्धिज्ञानप्राप्तिसर्वकर्मसंन्यासज्ञाननिष्ठाक्रमेणेति वाक्यशेषः । यस्तु पुनः अयुक्तः असमाहितः कामकारेण करणं कारः कामस्य कारः कामकारः तेन कामकारेण, कामप्रेरिततयेत्यर्थः, ‘मम फलाय इदं करोमि कर्म’ इत्येवं फले सक्तः निबध्यते । अतः त्वं युक्तो भव इत्यर्थः ॥ १२ ॥
In plain words
And since —
'The integrated one' is someone who stays composed, thinking, 'I do these actions for the Lord, not for anything I stand to gain' — having let go of the fruit of action, he reaches peace, the peace called liberation, the final peace — arriving there through purifying the inner being, gaining knowledge, giving up all action, and finally resting in that knowledge. That's what's implied, even though not spelled out. The 'unintegrated' one, though — the uncomposed one — acts under the compulsion of desire, driven by wanting, thinking, 'I do this for what I'll get out of it' — and clinging to the fruit like that, he stays bound. So: be integrated. That's the meaning.
Close to the Sanskrit
And because —
'The integrated one' — being composed, thinking, 'I do these actions for the Lord, not for my own fruit' — 'having abandoned the fruit of action,' having relinquished [it], 'attains peace,' called liberation, 'the final' — arising through the sequence of purified inner being, attained knowledge, renunciation of all action, and the discipline of knowledge — this is the implied remainder of the sentence. 'The unintegrated one,' however — [i.e.] uncomposed — 'by the compulsion of desire' — 'making' is 'doer,' the doer of desire is [called] 'the compulsion of desire' — by that compulsion of desire, [i.e.] being driven by desire, thinking, 'I do this action for my own fruit' — 'attached to the fruit, he is bound.' Therefore, be integrated. That is the meaning.