The White Hot Room is a realm that serves both as the afterlife and a base of operations for the Phoenix. There is where the Phoenix Work is done by the Phoenix. Rachel Summers refers to the White Hot Room as the Heart of the Phoenix. The White Hot Room also resides in the M'Krann Crystal which is a nexus of realities and contains a Neutron galaxy.
In the Avengers vs X-men story Arc at the 19th issue of Uncanny X-men, wwhen Cyclops has became the all powerful Dark Phoenix , he feels omnipotent and sometime while he is thionking some things he reaches the walls of the White Hot Room where he hears a voice which is Jean Grey from the White Hot Room saying that he is an idiot and tells him to tell Logan that she likes the name of his school.