In plain words
When the disciplined mind rests in Ātma alone, free from longing for all desires, then one is said to be collected.
Word for word
When the well-restrained mind rests in the Self alone, free from longing for all desired things, [one] is then called 'collected'.
The commentators
Bhagavadgītābhāṣyam
अथ अधुना कदा युक्तो भवति इत्युच्यते —
यदा विनियतं विशेषेण नियतं संयतम् एकाग्रतामापन्नं चित्तं हित्वा बाह्यार्थचिन्ताम् आत्मन्येव केवले अवतिष्ठते, स्वात्मनि स्थितिं लभते इत्यर्थः । निःस्पृहः सर्वकामेभ्यः निर्गता दृष्टादृष्टविषयेभ्यः स्पृहा तृष्णा यस्य योगिनः सः युक्तः समाहितः इत्युच्यते तदा तस्मिन्काले ॥ १८ ॥
In plain words
Now, at this point: when does a person actually become yukta — established in yoga? That is what is said next —
"When the mind is viniyata" — restrained in a particular way: reined in, held together, brought to a single point. Such a mind lets go of its running commentary on outside things and settles in the Self alone, the Self with nothing mixed into it; it finds, at last, a place to stand in itself. That is the sense. "Free of longing for every desire" — the yogin out of whom the reaching, the thirst, for objects seen and unseen has simply gone: he is the one called yukta, gathered, collected into one. Then — at that very moment.
Close to the Sanskrit
Now, at present, when does he become one who is yoked (yukta)? — thus it is stated —
When (yadā) — viniyatam: restrained in a special way, that is, held in, kept collected, come to one-pointedness — the mind, having abandoned thought of external objects, abides in the Self alone, the unmixed Self; it gains a standing in its own Self: this is the meaning. Niḥspṛhaḥ sarvakāmebhyaḥ: the yogin from whom longing, thirst, for objects seen and unseen has gone out — he is said to be yukta, that is, wholly gathered; then, at that time.