In the Srimad Bhagavatam 11.10.12 it is very nicely explained by Lord Sri Krishna to Uddhava:

ācāryo ‘raṇir ādyaḥ syād
ante-vāsy uttarāraṇiḥ
tat-sandhānaḿ pravacanaḿ
vidyā-sandhiḥ sukhāvahaḥ

“The spiritual master can be compared to the lower kindling stick, the disciple to the upper kindling stick, and the instruction given by the guru to the third stick placed in between. The transcendental knowledge communicated from guru to disciple is compared to the fire arising from the contact of these, which burns the darkness of ignorance to ashes, bringing great happiness both to guru and disciple.”