In plain words
Know one as a nitya-sannyāsin who neither hates nor desires; free from the pairs of opposites, mighty-armed one, one is easily released from bondage.
Word for word
He who neither hates nor desires is to be known [as] a constant renouncer; for, free from the pairs of opposites, O mighty-armed one, [he] is easily released from bondage.
The commentators
Bhagavadgītābhāṣyam
कस्मात् इति आह —
ज्ञेयः ज्ञातव्यः स कर्मयोगी नित्यसंन्यासी इति यो न द्वेष्टि किञ्चित् न काङ्क्षति दुःखसुखे तत्साधने च । एवंविधो यः, कर्मणि वर्तमानोऽपि स नित्यसंन्यासी इति ज्ञातव्यः इत्यर्थः । निर्द्वन्द्वः द्वन्द्ववर्जितः हि यस्मात् महाबाहो सुखं बन्धात् अनायासेन प्रमुच्यते ॥ ३ ॥
In plain words
"Why?" — and to that, he says:
'Should be known' means: should be understood that he, the man who follows the path of action, is the eternal renunciate — this is said of anyone who hates nothing and craves neither pain nor pleasure, nor the means to either. Someone like that, even while active in the world, should be recognized as the eternal renunciate — that is the meaning. 'Free of the pairs of opposites' means untouched by likes and dislikes, heat and cold, and all such pairs — because, being like this, mighty-armed one, he is set free from bondage easily, without any strain.
Close to the Sanskrit
"From what cause?" — thus (anticipating), he says:
'Should be known' — should be understood — he, the man of the path of action, is the eternal renunciate: so it is said of him who hates nothing and desires neither pain nor pleasure, nor the means to either. One of this kind, though engaged in action, should be known as the eternal renunciate — this is the meaning. 'Free of the pairs of opposites' — devoid of the pairs of opposites — for, since he is such, O mighty-armed one, he is released from bondage easily, without effort.