In order to put an end to the Kyuubi's rampage, the leader of the village, the Fourth Hokage, sacrificed his life and sealed the monstrous beast inside the newborn Naruto.
If you read that post already, then this next section is going to sound pretty familiar because we have a nearly identical situation with the word shounen.
Because seinen is aimed at an older demographic in comparison to its shonen counterpart, it often includes adult themes that you wouldn't generally see in a shonen title.