The lifestream is the promised land/heaven. And its also their equivalant of hell. Depends on how you've lived your life. When Tifa fell into it in Mideel she was surrounded by the memories of all the people she'd killed as a member of AVALANCHE. Something which she deeply regreted I guess. For the Cetra who were peaceful and in tune with the planet it is a place of rest and happiness.
Well at least that's what I took from the game anyway.