Bhashya
अरिष्टम् अविनाशिन कोश यथोक्त प्रपद्ये प्रपन्नोऽस्मि पुत्रायुषे ।अमुनामुनामुनेति त्रिर्नाम गृह्णाति पुत्रस्य ।तथा प्राण प्रपद्येऽमुनामुनामुना, भू प्रपद्येऽमुनामुनामुना, भुव प्रपद्येऽमुनामुनामुना, स्व प्रपद्येऽमुनामुनामुना, सर्वत्र प्रपद्ये इति त्रिर्नाम गृह्णाति पुन पुन ॥
In plain words
For the sake of the son's longevity, I take refuge in the indestructible and safe sheath as prescribed. He utters the son's name three times saying 'by such and such'. Similarly, for prana, earth, atmosphere, and heaven, he takes the name three times, repeating the invocation again and again.
Close to the Sanskrit
I take refuge in the safe, imperishable sheath as stated, for the son's life. He takes the son's name thrice with 'by such and such'. Likewise, 'I take refuge in prana by such and such', 'bhu by such and such', 'bhuvas by such and such', 'svar by such and such', 'I take refuge everywhere' - thus he takes the name thrice again and again.