Plains of Harmony refers the home and birthplace of the cosmic alicorns.
The universe is roughly a sphere containing organized stars and galaxies on the inside, and a "null space" devoid of light and thought on the outside. The Plains of Harmony are the inner edge of this sphere. Princess Luna describes it as being "beyond each and every one of the stars." While many alicorns still dwell there, a number of them set out across the cosmos to plant harmony and aid the spread of life. The spirits of the dead likewise return to the Plains that gave them life after they have passed.