The Phoenix Force is a cosmic force of destruction and rebirth. It destroys dying/diseased worlds and if it sees fit it will restart new life from it. Every writer seems to have their own interpretation so that's why readers are confused. In New X-Men she survives burning in the sun by releasing the phoenix consciousness and flies back to earth only to be killed??? In Endsong the Shiar show how wake the phoenix force to destroy it but only manage to shatter it into a million pieces. A piece of the phoenix manages to make it to earth and resurrect Jean's body. At the end of Endsong Jean fully merges with that piece and goes to the white hot room to look for the other pieces.
"I'm always Jean. And I'm always the Phoenix. I died. I scattered in a trillion directions. And then started to pull together again, outside the White Hot Room. But I'm starting to see now. Parts of me... Parts of me never came home."