Bhashya
दिव प्रत्यवभासमानम्- तत्, यत् आत्मानमेव परं पूर्णं ब्रह्म विदित्वा- अहम् अदः पूर्णं ब्रह्मास्मि इत्येवम्, पूर्णमादाय, तिरस्कृत्य अपूर्णस्वरूपताम् अविद्याकृतां नामरूपोपाधिसंपर्कजाम् एतया ब्रह्मविद्यया पूर्णमेव केवलम् अवशिष्यते ; तथा चोक्तम् 'तस्मात्तत्सर्वमभवत्' इति ।यः सर्वोपनिषदर्थो ब्रह्म, स एषः अनेन मन्त्रेण अनूद्यते, उत्तरसंबंधार्थम् ।ब्रह्मविद्यासाधनत्वेन हि वक्ष्यमाणानि साधनानि ओंकारदमदानदयाख्यानि विधित्सितानि, खिलप्रकरणसंबंधात् सर्वोपासनाङ्गभूतानि च ॥
In plain words
That radiant one shining in the sky -- that is what the word 'that' means. Once you know the Self itself as the one supreme, complete Brahman, you can say: I am that complete Brahman. Taking hold of that fullness this way strips away the false sense of incompleteness -- a sense ignorance produced, born only from contact with name and form. Through this knowledge of Brahman, fullness alone is left standing, nothing else. Scripture says the same thing elsewhere: therefore all this came to be full. What every Upaniṣad is really pointing to is Brahman, and this verse simply restates that, to connect what has been said with what comes next. The practices about to be taught -- meditation on Om, self-restraint, giving, compassion -- all serve knowledge of Brahman, belong to this part of the text, and support every form of meditative worship alike.
Close to the Sanskrit
[That is] the one shining forth in heaven -- that; in that, having known the Self alone as the supreme, full Brahman -- 'I am that full Brahman' -- thus, taking hold of fullness, having set aside the state of incomplete nature made by ignorance, born from contact with the limiting adjuncts of name and form, by this knowledge of Brahman fullness alone, exclusively, remains. And thus it is said: 'therefore all that came to be' -- thus. That which is the meaning of all the Upaniṣads, Brahman -- that very one is recapitulated by this mantra, for the sake of connection with what follows. Because, as a means to the knowledge of Brahman, the means about to be stated -- called Om, restraint, giving, and compassion -- are intended to be enjoined, and, because of their connection with the supplementary section, they are also limbs common to every form of meditative worship.