Bhashya
मानवो ब्रह्मा मौनाचरणान्मननाद्वा ज्ञानवत्त्वात् ; ततो ब्रह्मैवैकः ऋत्विक् कुरून् कर्तॄन्—योद्धॄनारूढानश्वा बडबा यथा अभिरक्षति, एवंवित् ह वै ब्रह्मा यज्ञं यजमानं सर्वांश्च ऋत्विजोऽभिरक्षति, तत्कृतदोषापनयनात् ।यत एवं विशि- ष्टो ब्रह्मा विद्वान्, तस्मादेवंविदमेव यथोक्तव्याहृत्यादिविदं ब्रह्माणं कुर्वीत, नानेवंविदं कदाचनेति ।द्विरभ्यासोऽध्याय- परिसमाप्त्यर्थः ॥
In plain words
The sage Manava is called Brahman because of his practice of silence, deep contemplation, and wisdom. Therefore, just as a mare fiercely protects her young and mounted warriors, this single wise Brahman priest safeguards the sacrifice, the patron, and all the officiating priests by instantly clearing away any ritual flaws they commit. Because he is thus uniquely qualified and wise, one must appoint only a priest who possesses this knowledge—and understands the mystical utterances as taught—to the office of Brahman, and never, under any circumstances, one who lacks it. The repetition of this injunction marks the conclusion of the chapter.
Close to the Sanskrit
The Manava is the Brahman because of the practice of silence, reflection, or possession of knowledge; therefore Brahman alone as the single priest protects the performers and warriors mounted on horses just as a mare protects them—so the wise Brahman indeed protects the sacrifice, the patron, and all the priests by removing the faults committed by them. Since the Brahman is thus distinguished and wise, therefore one should appoint only a priest knowing thus and knowing the vyahritis as stated to the office of Brahman, never one not knowing thus at any time. The doubling of the phrase is for the purpose of concluding the chapter.