Bhashya
तं पुत्रं ह उवाच- वटधानायां भिन्नायां यं वटबीजाणिमानं हे सोम्य एतं न निभालयसे न पश्यसि, तथा अप्येतस्य वै किल सोम्य एष महान्न्यग्रोधः बीजस्य अणिम्नः सूक्ष्मस्य अदृश्यमानस्य कार्यभूतः स्थूलशाखास्कन्धफलपलाशवान् तिष्ठति उत्पन्नः सन्, उत्तिष्ठतीति वा, उच्छब्दोऽध्याहार्यः ।अतः श्रद्धत्स्व सोम्य सत एव अणिम्नः स्थूलं नामरूपादिमत्कार्यं जगदुत्पन्नमिति ।यद्यपि न्यायागमाभ्यां निर्धारितोऽर्थः तथैवेत्यवगम्यते, तथापि अत्यन्तसूक्ष्मेष्वर्थेषु बाह्यविषयासक्तमनसः स्वभावप्रवृत्तस्यासत्यां गुरुतरायां श्रद्धायां दुरवगमत्वं स्यादित्याह- श्रद्धत्स्वेति ।श्रद्धायां तु सत्यां मनसः समाधानं बुभुत्सितेऽर्थे भवेत्, ततश्च तदर्थावगतिः, 'अन्यत्रमना अभूवम्' इत्यादिश्रुतेः ॥
In plain words
The father said to his son: When you split the banyan seed, the subtle essence within it is not seen by you; yet from that very invisible subtlety rises this great banyan tree, with its thick trunk, branches, leaves, and fruit. Therefore, my dear, have faith. Understand that this entire gross world of name and form springs from that subtle Existent alone. Even though this truth is established by logic and scripture, it remains hard to grasp for those whose minds are attached to external objects and lack deep faith. Hence the instruction: 'Have faith.' For when there is faith, the mind becomes stilled in the truth to be known, and true understanding dawns.
Close to the Sanskrit
He said to that son: when the banyan grain is split, which banyan seed subtlety, O dear, you do not perceive, do not see, yet verily from this subtlety of the seed, unseen, being an effect possessing thick branches, trunk, fruits, and leaves, or rising up (the word 'ut' is to be supplied), this great banyan tree stands. Therefore have faith, O dear, that the gross universe of name and form has originated from the Existent alone, which is subtlety. Although the meaning ascertained by logic and scripture is understood just thus, nevertheless in extremely subtle matters, for one whose mind is attached to external objects and naturally inclined to them, in the absence of deep faith, comprehension would be difficult—hence he said 'have faith'. When faith is present, the settling of the mind in the object to be known takes place, and thence the comprehension of that meaning, from the shruti text beginning with 'My mind was elsewhere', etc.