In plain words
In that state the mind, restrained by the practice of yoga, becomes quiet, and one beholds Ātma through the purified mind and is satisfied in Ātma alone.
Word for word
Where the mind, restrained by the practice of yoga, comes to rest, and where, seeing the Self by the mind, [one] is satisfied in the Self alone,
The commentators
Bhagavadgītābhāṣyam
एवं योगाभ्यासबलादेकाग्रीभूतं निवातप्रदीपकल्पं सत् —
यत्र यस्मिन् काले उपरमते चित्तम् उपरतिं गच्छति निरुद्धं सर्वतो निवारितप्रचारं योगसेवया योगानुष्ठानेन, यत्र चैव यस्मिंश्च काले आत्मना समाधिपरिशुद्धेन अन्तःकरणेन आत्मानं परं चैतन्यं ज्योतिःस्वरूपं पश्यन् उपलभमानः स्वे एव आत्मनि तुष्यति तुष्टिं भजते ॥ २० ॥
In plain words
So the mind, made one-pointed by the strength of yoga practice, becomes steady like a lamp flame in a place with no wind —
'Where' means: at the moment when the mind comes to rest — when it goes still, held in, its running out toward things blocked from every side — and it is the doing of yoga, not the admiring of it, that holds it. 'And where' means: at that same moment, when with the self — that is, with an inner instrument scoured clean by samādhi — he sees the Self, and seeing here means actually meeting it: supreme consciousness, light itself, nothing added. And there, in his own Self alone, he is content; contentment is what he comes into. (20)
Close to the Sanskrit
Thus, having become one-pointed by the power of the practice of yoga, like a lamp in a windless place, being so —
'Where' — that is, at which time — 'the mind comes to rest', that is, it goes to cessation, 'restrained', that is, with its ranging warded off on every side, 'by the pursuit of yoga', that is, by the performance of yoga; and 'where too' — that is, and at which very time — 'by the self', that is, by the inner instrument thoroughly purified by samādhi, 'seeing', that is, directly apprehending, 'the Self', the supreme consciousness whose very nature is light, 'he is content in his own Self alone', that is, he comes into contentment. (20)