Bhashya
मनोमय ईश्वर उक्त आकाशात्मेति च ब्रह्मणो गुणैक-देशत्वेन ।अथेदानीं मनआकाशयोः समस्तब्रह्मदृष्टिविधा-नाथ आरम्भ मनो ब्रह्मेत्यादि ।मन मनुतेऽनेनेत्यन्त-करणं तद्ब्रह्म परमित्युपासीतेति एतदात्मविषयं दर्शनम्अध्यात्मम् ।अथ अधिदैवतं देवताविषयमिदं वक्ष्याम ।आकाशो ब्रह्मेत्युपासीत, एवमुभयमध्यात्ममधिदैवतं च उभयब्रह्मदृष्टिविषयम् आदिष्टम् उपदिष्टं भवति, आकाशमनसोःसूक्ष्मत्वात् मनसोपलभ्यत्वाच्च ब्रह्मणः, योग्यं मनो ब्रह्म-दृष्टे, आकाशश्च, सर्वगतत्वात्सूक्ष्मत्वादुपाधिहीनत्वाच्च ॥
In plain words
The Lord has been described as mind-formed, having space for His very self, sharing an aspect of Brahman. Now, to establish a unified contemplation of Brahman in both mind and space, the teaching begins with 'Mind is Brahman.' The mind is the inner instrument of thought; meditating on it as the supreme Brahman is the inner path. Now for the cosmic path: meditate on space as Brahman. Thus both inner and outer meditations on Brahman are taught. Because mind and space are subtle, and because Brahman is apprehended through the mind, both mind and space are fitting supports for the vision of Brahman—space especially because it is all-pervading, subtle, and free from gross limitations.
Close to the Sanskrit
The mind-made Lord has been spoken of as having space as His self, by way of sharing a partial characteristic of Brahman. Now, to enjoin a unified vision of Brahman regarding mind and space, the text begins 'mind is Brahman' etc. Mind means the inner instrument by which one thinks; one should meditate upon that supreme Brahman—this is the contemplative perspective pertaining to the self. Now we shall speak of that pertaining to the deities: one should meditate upon space as Brahman. Thus both are taught as having the vision of Brahman as their object, both the inner and the cosmic, because of the subtlety of mind and space, and because Brahman is apprehended by the mind. Hence mind is suitable for the vision of Brahman, and so is space, because of its all-pervading nature, subtlety, and absence of limitations.