SB 11.12.18

yathānalaḥ khe ’nila-bandhur uṣmā
 balena dāruṇy adhimathyamānaḥ
aṇuḥ prajāto haviṣā samedhate
 tathaiva me vyaktir iyaṁ hi vāṇī
Synonyms: 
yathā — just as; analaḥ — fire; khe — in the space within wood; anila — air; bandhuḥ — whose help; uṣmā — heat; balena — strongly; dāruṇi — within the wood; adhimathyamānaḥ — being kindled by friction; aṇuḥ — very tiny; prajātaḥ — is born; haviṣā — with ghee (clarified butter); samedhate — it increases; tathā — similarly; eva — indeed; me — My; vyaktiḥ — manifestation; iyam — this; hi — certainly; vāṇī — the Vedic sounds.
Translation: 
When sticks of kindling wood are vigorously rubbed together, heat is produced by contact with air, and a spark of fire appears. Once the fire is kindled, ghee is added and the fire blazes. Similarly, I become manifest in the sound vibration of the Vedas.
Purport: 

Lord Kṛṣṇa here explains the most confidential meaning of Vedic knowledge. The Vedas first regulate ordinary material work and channel the fruits into ritualistic sacrifices, which ostensibly reward the performer with future benefits. The real purpose of these sacrifices, however, is to accustom a materialistic worker to offering the fruits of his work to a superior Vedic authority. An expert fruitive worker gradually exhausts the possibilities of material enjoyment and naturally gravitates toward the superior stage of philosophical speculation on his existential situation. By increased knowledge, one becomes aware of the unlimited glories of the Supreme and gradually takes to the process of loving devotional service to the transcendental Absolute Truth. Lord Kṛṣṇa is the goal of Vedic knowledge, as the Lord states in Bhagavad-gītā: vedaiś ca sarvair aham eva vedyaḥ. The Lord gradually becomes manifest in the progression of Vedic rituals, just as fire is gradually manifest by the rubbing of firewood. The words haviṣā samedhate (“the fire increases by addition of ghee”) indicate that by the progressive advancement of Vedic sacrifice, the fire of spiritual knowledge gradually blazes, illuminating everything and destroying the chain of fruitive work.

Lord Kṛṣṇa considered Uddhava to be the most qualified person to hear this elaborate transcendental knowledge; therefore the Lord mercifully instructs Uddhava so that he may enlighten the sages at Badarikāśrama, thus fulfilling the purpose of the sages’ lives.