Nephilim is a contemporary role-playing game of the occult, first written in French by Frédéric Weil and Fabrice Lamidey, and published by MultiSim in 1992. The game was translated to English (and adapted in the process) by Sam Shirley and Greg Stafford, and published in 1994 by Chaosium Games. Eight supplements were published.
Loosely translating from the French description on MultiSim's web page (which has disappeared): "Nephilim is a comtemporary occult role-playing game. Players adopt the role of mystic beings composed of magical elemental fields (Air, Water, Fire, Earth, and Moon). Mysteriously, the Nephilim are now awakening en masse after a long slumber, incarnating in humans, and continuing to acquire occult knowledge. Their ultimate goal is to attain a superior consciousness called Agartha. They face numerous dangers, the principle threat being secret societies, the most determined being the Templars."
There is no one true answer, at least with regard to the specifics. Like any roleplaying game, it's what you make of it. Ultimately, Nephilim is a game is about secrets, mysteries both of the self and of the world, of the esoteric and the everyday. In short, it's about discovery and revelation.
Stores which carry roleplaying games are unlikely to have the rulebook and supplements from Chaosium Games. For a nominal shipping and handling charge, you can order from Chaosium directly; PDFs can also be acquired via DriveThruRPG.