After you defend Whiterun from the Dragon outside its gates, you hear the Greybeards' call from atop High Hrothgar. After you return to Jarl Balgruuf, he explains that they are calling for the Dragonborn.
The path to High Hrothgar is dangerous and unforgiving, so what does the Jarl do for you? Gives you a single woman to help you out on your trip.
Once again, thank you Jarl.
