It means the one who holds the reins.
In the Mahabharata, Krishna does not fight at Kurukshetra. He drives. The sarathi knows the ground, reads the weather, picks the line and keeps the chariot steady so the person riding can look up and see where they are.
That is the whole idea behind the company, and it is a fair description of what a good tour operator actually does. You should be looking out of the window. Somebody else should be worrying about whether the pass is open.
