Jason's father Aeson was the rightful ruler of Iolcos in Thessaly. But he was overthrown by his wicked half-brother Pelias, who proceeded to kill every one of Aeson's descendants he could lay his hands on. This was to ensure that his tyranny would not be challenged by anyone seeking to reestablish Aeson's line.
Although Pelias spared the life of Aeson, any of his children would be in immediate danger of being killed. Aeson's wife Alcimede realized this, and when she gave birth to Jason, she immediately tried to protect him.
First of all, she had her female attendants pretend that the infant Jason was stillborn. But as such a pretense could only be kept up for so long, Alcimede arranged for Jason to be sent away and reared by Chiron the centaur, a creature with the upper body of a man and the lower body of a horse.
In ancient Greek mythology, Chiron had the reputation of being the wisest centaur, which made him ideally placed to raise children. As well as raising Jason, he also educated the legendary warrior Achilles and trained Hercules, the famous Greek hero. Thanks to Chiron, Jason grows into a wise young man ready and able to come into his inheritance by stealing the Golden Fleece.