In ancient Greece... the god Apollo was very popular. He was the god of music, the sun, and prophecy - he could see glimpses of the future. But people were always asking him about their futures, and he had other obligations. And so Apollo decided he needed someone else with his gifts.
He found a young priestess, and told her of his offer. She accepted - who would not want to be Apollo's priestess?
Apollo gifted the priestess with the power to glimpse the future, though she could only tell the truth about it. She was unable to lie. He created a temple for her in the city of Delphi, which was said to be at the center of the world, and gave her a few priests to assist with running the place. She assumed the title Pythia.
The Pythia resided behind a curtain, sitting on a 3-legged stool while gas poured out of the ground.
Often she spoke in incomprehensible mutters that the assisting priests had to interperate what had been said - naturally, there were contradictions.
She used her gifts to deliver prophecies, or tell bits of the future to anyone who asked a direct question.
Still, people from far and wide came to seek the truth from the Oracle of Delphi, whether rich or poor, king or peasant.
Sometimes the offer was too great, and the priests let them cut.
The Pythia advice became so high in demand that people would offer bribes to get to the front of the line.