Bhashya
अथ यत्सत्रायणमित्याचक्षते, ब्रह्मचर्यमेव तत् ।तथा मतः परस्मादात्मनः आत्मनस्त्राणं रक्षणं ब्रह्मचर्यसाधनेन विन्दते ।अतः सत्रायणशब्दमपि ब्रह्मचर्यमेव तत् ।अथ यन्मौनमित्याचक्षते, ब्रह्मचर्यमेव तत् ; ब्रह्मचर्येणैव साधनेन युक्तः सन् आत्मानं शास्त्राचार्याभ्यामनुविद्य पश्चात् मनुते ध्यायति ।अतो मौनशब्दमपि ब्रह्मचर्यमेव ॥
In plain words
Now, whatever is called 'satrāyaṇa' is truly nothing other than brahmacarya. Through the practice of brahmacarya, a person secures the protection and rescue of the self from the supreme Self; therefore, the very word 'satrāyaṇa' means brahmacarya. And whatever is called 'mauna' (silence) is also brahmacarya. Equipped with brahmacarya as the sole instrument, and having realized the self through scripture and teacher, one subsequently reflects and contemplates. Therefore, the word 'mauna' likewise refers to brahmacarya alone.
Close to the Sanskrit
Now that which they call satrāyaṇa, that is brahmacarya indeed. Thus one finds the rescue and preservation of the self from the supreme self through the means of brahmacarya. Therefore the word satrāyaṇa is also brahmacarya indeed. Now that which they call mauna, that is brahmacarya indeed; being equipped with brahmacarya alone as the means, having thoroughly known the self through scripture and teacher, afterwards one reflects and contemplates. Therefore the word mauna is also brahmacarya.