In plain words
The great brahman, Prakṛti, is My womb; in it I place the seed. From that comes the birth of all beings, Bharata.
Word for word
My womb [is] the great brahman; in it I place the seed; the birth of all beings comes about from that, O Bhārata.
मम योनिर्महद् ब्रह्म तस्मिन् गर्भं दधाम्यहम्सम्भवः सर्वभूतानां ततो भवति भारत
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
क्षेत्रक्षेत्रज्ञसंयोगः ईदृशः भूतकारणम् इत्याह —
मम स्वभूता मदीया माया त्रिगुणात्मिका प्रकृतिः योनिः सर्वभूतानां कारणम् । सर्वकार्येभ्यो महत्त्वात् भरणाच्च स्वविकाराणां महत् ब्रह्म इति योनिरेव विशिष्यते । तस्मिन् महति ब्रह्मणि योनौ गर्भं हिरण्यगर्भस्य जन्मनः बीजं सर्वभूतजन्मकारणं बीजं दधामि निक्षिपामि क्षेत्रक्षेत्रज्ञप्रकृतिद्वयशक्तिमान् ईश्वरः अहम् , अविद्याकामकर्मोपाधिस्वरूपानुविधायिनं क्षेत्रज्ञं क्षेत्रेण संयोजयामि इत्यर्थः । सम्भवः उत्पत्तिः सर्वभूतानां हिरण्यगर्भोत्पत्तिद्वारेण ततः तस्मात् गर्भाधानात् भवति हे भारत ॥ ३ ॥
In plain words
The joining of the field with the knower of the field is of just this nature: it is the cause of beings. And so he says —
This nature — māyā, made of the three guṇas, prakṛti itself — belongs entirely to Me; it's My own. It functions as the womb, the source, the very cause of every being there is. And because it towers over everything it produces, and because it sustains all its own creations, this same womb earns two great names: 'the great one,' and 'Brahman.' Into that great womb, I, the Lord, holding both powers — the power of the field and the power of its knower — place a seed: the seed for the birth of Hiraṇyagarbha, the cosmic being, the very seed from which every creature's birth follows. In other words, I join the field-knower, shaped as it is by ignorance, desire, and action, to the field itself. Everything that comes to exist, every being's arising, happens through the gateway of Hiraṇyagarbha's own arising — and that, in turn, happens from this very act of Mine, placing the seed. That's how it all begins, Bhārata.
Close to the Sanskrit
The conjunction of the field and the knower of the field is of this kind — the cause of beings: thus he says —
My own, mine, māyā, consisting of the three guṇas, prakṛti, is the womb, the cause of all beings. Because of its greatness relative to all its effects, and because of sustaining its own modifications, that very womb is distinguished by the names 'the great one' and 'Brahman'. In that great Brahman, that womb, I, the Lord, possessed of the twin powers of kṣetra and kṣetrajña, place, deposit, the germ — the seed for the birth of Hiraṇyagarbha, the seed that is the cause of the birth of all beings — that is, I join the kṣetrajña, which conforms in its nature to the limiting conditions of ignorance, desire, and action, with kṣetra: that is the meaning. The origination, the arising, of all beings occurs through the gateway of Hiraṇyagarbha's own arising, from that, from that very depositing of the germ, O Bhārata.