When humans left Earth in search of a new planet after the war on Earth, they entered cryosleep for more than a century. The Titan Anicetus was equipped with what could be considered "passive" A.I., to maintain the mission to travel and maintain the ship's systems and destination. During cryosleep, this A.I. entered an "active" state, where it began selecting certain humans, specifically the young, for what would be considered "Titan Hosts". This augmentation eventually killed them in cryosleep, only to be brought to life once Anicetus reached Acheron orbit. Consider the analogy like "a hard restart" for these individuals. These Titan-Born were augmented to the point where their lifespan is almost limitless and have, simply put, a rewired connection to directly control and communicate with Titan Terraform Machines. Alithea and Amyntas were among those chosen during cryosleep, along with eight others (one for each Titan).
When Anicetus first arrived at Acheron, everyone woke from cryosleep except the Titanborn. While waiting for Anicetus to finish terraforming Acheron, the surviving humans created a religion around the Titanborn, "those bound by frost". It was only when the terraforming finally completed that the first Titanborn, Amyntas, woke.
In a more abstract sense, Titan-Born also relates to one who strips themselves of all human superstitions and barbaric traits. One who understands that gods are non-existent simply due to the basic logic that gods are man-made. Therefore, man and god are the same and in the end, the enemy to "human evolution". This evolution requires the acceptance of machine augmentation. Can something human still be salvaged once this evolution occurs?