Bhashya
विद्येत्युच्यते ; इदं सर्वम्— यस्मात् ब्रह्मणो विज्ञानात्सर्वं भवति ॥
In plain words
This is what we mean by knowledge: once Brahman is realized, everything else is realized along with it -- which is exactly why the verse says, 'this is everything.'
Close to the Sanskrit
This [teaching] is called knowledge. [As for] 'this is everything' -- [it is said] because, from the realization of Brahman, everything comes to be [known].