Bhashya
तं ह उवाच पुनः पिता—यथा सोम्य महतः अभ्या-हितस्येत्यादि समानम्, एकमङ्गारं शान्तस्याग्नेः खद्योत-मात्रं परिशिष्टं तं तृणैश्चूर्णैश्च उपसमाधाय प्राज्वलयेत्वर्धयेत् ।तेनेद्धेन अङ्गारेण ततोऽपि पूर्वपरिमाणात् बहुदहेत् ॥
In plain words
The father spoke to him again: 'My dear, the beginning is just as before—when a large fire has died down until only a single ember the size of a firefly remains, if you cover it with dry grass and powdered fuel, you can rekindle it. And from that single burning coal, it will flare up and burn even more fiercely than before.'
Close to the Sanskrit
The father spoke to him again: Just as, O dear one, of a great piled fire and so on is identical, a single ember of a subdued fire firefly-sized is left over, having covered that with grasses and powders one should ignite it, one should grow it. By that kindled ember from that even more than the prior measure it would burn.