In plain words
One endowed with yoga, pure in mind, master of the body and with senses conquered, whose Ātma has become the Ātma of all beings, is not tainted even while acting.
Word for word
Endowed with yoga, pure in mind, master of the body, with senses conquered, whose self has become the self of all beings, [he] is not tainted even while acting.
The commentators
Bhagavadgītābhāṣyam
यदा पुनः अयं सम्यग्ज्ञानप्राप्त्युपायत्वेन —
योगेन युक्तः योगयुक्तः, विशुद्धात्मा विशुद्धसत्त्वः, विजितात्मा विजितदेहः, जितेन्द्रियश्च, सर्वभूतात्मभूतात्मा सर्वेषां ब्रह्मादीनां स्तम्बपर्यन्तानां भूतानाम् आत्मभूतः आत्मा प्रत्यक्चेतनो यस्य सः सर्वभूतात्मभूतात्मा सम्यग्दर्शीत्यर्थः, स तत्रैवं वर्तमानः लोकसङ्ग्रहाय कर्म कुर्वन्नपि न लिप्यते न कर्मभिः बध्यते इत्यर्थः ॥ ७ ॥
In plain words
But when this is taken up instead as the means to attaining right knowledge —
'Endowed with yoga' means just that; 'purified in mind' means purified within; 'of conquered self' means having mastered the body; 'of conquered senses'; and 'whose self has become the self of all beings' — of every being, from Brahmā down to a blade of grass — means one whose own inward awareness has become their very self. In other words: one who sees truly. Being like this, even while acting for the good of the world, he isn't tainted, isn't bound by what he does. That's the meaning.
Close to the Sanskrit
But when this one, by way of being the means to the attainment of right knowledge —
'Endowed with yoga' — endowed with yoga; 'purified in mind' — [i.e.] purified in inner being; 'of conquered self' — [i.e.] of conquered body; 'of conquered senses,' and 'whose self has become the self of all beings,' of all beings from Brahmā down to a blade of grass, [i.e.] whose inward consciousness has become their self — he who is of such a nature is 'whose self has become the self of all beings,' meaning one who sees rightly. That is the meaning. Being thus situated, although performing action for the guidance of the world, he is not tainted, is not bound by his actions. That is the meaning.