Bhashya
एष प्रजापतिः यद्धृदयं प्रजापतिः अनुशास्तीत्यनन्तरमे-वाभिहितम् ।कः पुनरसौ अनुशास्ता प्रजापतिरित्युच्यते-एष प्रजापतिः; कोऽसौ? यद्धृदयम्, हृदयमिति हृदयस्थाबुद्धिरुच्यते ; यस्मिन् शाकल्यब्राह्मणान्ते नामरूपकर्मणामु-पसंहार उक्तो दिग्विभागद्वारेण, तदेतत् सर्वभूतप्रतिष्ठं सर्व-भूतात्मभूतं हृदयं प्रजापतिः प्रजानां स्रष्टा ; एतत् ब्रह्म, बृह-त्त्वात् सर्वात्मत्वाच्च ब्रह्म ; एतत्सर्वम् ; उक्तं पञ्चमाध्यायेहृदयस्य सर्वत्त्वम् ; तत्सर्वं यस्मात् तस्मादुपास्यं हृदयं ब्रह्म ।
In plain words
This is Prajāpati — the heart is Prajāpati, ruling from within. That much was just said. But who, exactly, is this ruler called Prajāpati? Here is the answer: this is Prajāpati. Which one? The heart — and by 'heart' is meant the understanding that lives there, the very intellect seated within you. This is the same reality that, at the close of the teaching on Śākalya, was shown to gather every name, every form, every action back into itself through the map of the directions. That heart is the ground every being stands on; it has become the very self of every being; it is Prajāpati, the one who brings creatures into existence. This is Brahman — vast, and the self of everything, and for that very reason Brahman. This is everything. The fifth chapter already said the heart is all. Because it is all, the heart — which is Brahman — is what you are to meditate on.
Close to the Sanskrit
This is Prajāpati; that which is the heart, Prajāpati rules — this was stated immediately before. But who indeed is this ruler called Prajāpati? It is explained: 'This is Prajāpati' — who is this? 'Which is the heart' — the word 'heart' here means the intellect seated in the heart. That same heart concerning which, at the end of the Śākalya brāhmaṇa, the summation of names, forms and deeds was declared by way of the division of quarters — that heart, the support of all beings, become the self of all beings, is Prajāpati, the creator of creatures. This is Brahman, because of its vastness and because it is the self of all — Brahman. This is everything; it was said in the fifth chapter that the heart is all. Since that heart is everything, therefore the heart, which is Brahman, is to be meditated upon.