[DustyPorViva]

Well if you're joking, then, it means when God said let there be an NPC.
If you're not, then it means exactly what Rapidwolve said. The first time the NPC is 'loaded'(first time a/the[not sure] player enters the level), and is not to be confused with onPlayerenters, which executes each time a/the player enters the level, or like mentioned, the NPC/level is updated.