The Catalyst is a person whose existence makes changes in the course of history possible. He can only be recognised by the White Prophet of that period, who makes prophecies about how the future must be and guides the Catalyst to change the course of history.
Relation between the White Prophet and the Catalyst
The relation between the White Prophet and the Catalyst is a complex one, because while changing the course of history, the White Prophet might have to ask some sacrificies from the Catalyst. This might cause some friction between the two and there can also be some jealousy between them as the story of Redda and Hoquin the White his rabbit illustrates:
Redda was a very young woman, little more than a child. She didn't like Hoquin's rabbit who always tried to drive her away from Hoquin. The day the rabbit died, Redda made food of it but did not tell Hoquin about it until after he had dined upon it.
White Prophets and their Catalysts