In plain words
For the sage wishing to ascend to yoga, action is said to be the means; for that same sage, once established in yoga, cessation from all action is said to be the means.
Word for word
For the sage wishing to ascend to yoga, action is said [to be] the means; for that same [sage], having ascended to yoga, cessation is said [to be] the means.
The commentators
Bhagavadgītābhāṣyam
एवं परमार्थसंन्यासकर्मयोगयोः कर्तृद्वारकं संन्याससामान्यमपेक्ष्य ‘यं संन्यासमिति प्राहुर्योगं तं विद्धि पाण्डव’ इति कर्मयोगस्य स्तुत्यर्थं संन्यासत्वम् उक्तम् । ध्यानयोगस्य फलनिरपेक्षः कर्मयोगो बहिरङ्गं साधनमिति तं संन्यासत्वेन स्तुत्वा अधुना कर्मयोगस्य ध्यानयोगसाधनत्वं दर्शयति —
आरुरुक्षोः आरोढुमिच्छतः, अनारूढस्य, ध्यानयोगे अवस्थातुमशक्तस्यैवेत्यर्थः । कस्य तस्य आरुरुक्षोः ? मुनेः, कर्मफलसंन्यासिन इत्यर्थः । किमारुरुक्षोः ? योगम् । कर्म कारणं साधनम् उच्यते । योगारूढस्य पुनः तस्यैव शमः उपशमः सर्वकर्मभ्यो निवृत्तिः कारणं योगारूढस्य साधनम् उच्यते इत्यर्थः । यावद्यावत् कर्मभ्यः उपरमते, तावत्तावत् निरायासस्य जितेन्द्रियस्य चित्तं समाधीयते । तथा सति स झटिति योगारूढो भवति । तथा चोक्तं व्यासेन — ‘नैतादृशं ब्राह्मणस्यास्ति वित्तं यथैकता समता सत्यता च । शीलं स्थितिर्दण्डनिधानमार्जवं ततस्ततश्चोपरमः क्रियाभ्यः’(मो. ध. १७५ । ३७) इति ॥ ३ ॥
In plain words
So, on the basis of what transcendent renunciation and the yoga of action share through the agent, Krishna said: 'what they call renunciation, know that to be yoga too' — ascribing renunciant-status to the yoga of action purely to praise it. Having praised it this way, as the fruit-indifferent, external support for the yoga of meditation, he now goes on to show how the yoga of action actually serves as the means to reach the yoga of meditation—
'Of one desiring to ascend' means exactly that — one still trying to climb toward it. 'Of one who has not yet ascended' means someone still unable to actually stay settled in meditation. That's the meaning. Who is this person 'desiring to ascend'? The sage — someone who has already renounced the fruit of action. Ascending to what? To yoga. 'Action is called the means' for him. But for someone who has already ascended in yoga, it's different — for him alone, tranquility, ceasing from all actions, is now called the means. That's the meaning. The more one withdraws from action, the more the mind of someone unstrained, senses mastered, gathers into composure. And once that happens, he quickly becomes one who has truly ascended in yoga. As Vyāsa himself put it: 'the brahmin has no wealth like this — oneness, equanimity, truthfulness, good conduct, steadiness, laying down every impulse to harm, uprightness, and simply ceasing from restless activity.'
Close to the Sanskrit
Thus, having regard to the commonality of renunciation, by way of the agent, of transcendent renunciation and the yoga of action — 'which they call renunciation, know that to be yoga, Pāṇḍava' — renunciant-status was ascribed to the yoga of action for the sake of praising it. Having praised it thus, by [calling it] renunciant-status, as the external means, indifferent to fruit, of the yoga of meditation — now he shows the yoga of action's being the means to the yoga of meditation—
'Of one desiring to ascend' — [i.e.] of one desiring to ascend — 'of one who has not [yet] ascended,' precisely of one who is still incapable of abiding in the yoga of meditation. That is the meaning. Of whom, that one 'desiring to ascend'? 'Of the sage,' [i.e.] the renouncer of the fruit of action. That is the meaning. Desiring to ascend to what? To yoga. 'Action is said to be the means,' [i.e.] the means. 'Of one who has ascended in yoga,' however, for him alone — 'tranquility,' [i.e.] cessation, withdrawal from all actions — is said to be the means for one who has ascended in yoga, [i.e.] the means. That is the meaning. To whatever extent one desists from actions, to that same extent the mind of one without exertion, whose senses are conquered, becomes composed. This being so, he quickly becomes 'one who has ascended in yoga.' And thus it has been said by Vyāsa: 'the brahmin has no wealth like this: unity, equanimity, truthfulness, good conduct, steadfastness, laying aside the rod [of harm], uprightness, and ceasing from [restless] activities' (Mokṣadharma 175.37).