In plain words
Know that severance from union with sorrow to be called yoga. This yoga must be practiced with determination and an undiscouraged mind.
Word for word
One should know that severance from contact with sorrow [as] called yoga; that yoga is to be practised with resolve, with an undejected mind.
The commentators
Bhagavadgītābhāṣyam
‘यत्रोपरमते’(भ. गी. ६ । २०) इत्याद्यारभ्य यावद्भिः विशेषणैः विशिष्ट आत्मावस्थाविशेषः योगः उक्तः —
तं विद्यात् विजानीयात् दुःखसंयोगवियोगं दुःखैः संयोगः दुःखसंयोगः, तेन वियोगः दुःखसंयोगवियोगः, तं दुःखसंयोगवियोगं योग इत्येव संज्ञितं विपरीतलक्षणेन विद्यात् विजानीयादित्यर्थः । योगफलमुपसंहृत्य पुनरन्वारम्भेण योगस्य कर्तव्यता उच्यते निश्चयानिर्वेदयोः योगसाधनत्वविधानार्थम् । स यथोक्तफलो योगः निश्चयेन अध्यवसायेन योक्तव्यः अनिर्विण्णचेतसा न निर्विण्णम् अनिर्विण्णम् । किं तत् ? चेतः तेन निर्वेदरहितेन चेतसा चित्तेनेत्यर्थः ॥ २३ ॥
In plain words
Beginning with "where the mind comes to rest" (6.20) and the lines that follow, yoga has been stated: a particular state of the self, marked out by every one of the qualifications given there —
One should understand this: yoga is disconnection from union with sorrow. Union with sorrows is called ‘union with sorrow’; breaking free of that is ‘disunion from union with sorrow.’ That very disunion from sorrow is what's called yoga—understood, you could say, by defining it in reverse, through what it isn't. Having summed up yoga's fruit, the text now takes the subject up again, to insist on the necessity of practicing it—so as to establish resolve and freedom from despondency as the actual means to yoga. That yoga, bearing the fruit already described, must be practiced with resolve, with real determination, with a mind that hasn't given up—not despondent. What kind of mind is that? A mind simply free of despondency.
Close to the Sanskrit
Beginning with 'where it comes to rest' (Bh. Gī. 6.20) and so forth, yoga — a particular state of the self, qualified by however many qualifiers [were given there] — has been spoken of —
That—namely the disunion from union with sorrow—one should know, should understand: union with sorrows is ‘union with sorrow’; disunion from that is ‘disunion from union with sorrow.’ That disunion from union with sorrow is indeed called ‘yoga,’ and one should know, understand it through a definition stated in reverse terms—that is the meaning. Having summed up the fruit of yoga, again, by a fresh undertaking, the necessity of practicing yoga is stated, for the sake of enjoining resolve and freedom from despondency as the means to yoga. That yoga, whose fruit is as already described, is to be practiced with resolve, i.e. with determination, with an undespondent mind—not despondent. What is that? Mind—with that mind devoid of despondency—that is the meaning.