The Hallow of Eru was atop the great mountain of Meneltarma in the center of the island of Númenor.

The hallow was a place where only the Kings of Númenor would go and it was the only temple in all of the land. There was a tradition of the kings bringing the 'first fruits' of the year there as an offering to Ilúvatar.
Encyclopedia entry originally written by Nienna-of-the-Valar