This is wonderful, emotional piece and I have very much enjoyed playing it on an instrument with the timbres and registrations that Franck would have employed.
It has a number of recurring themes including sections of restless, melanchoic ever-changing tonality built on a rippling left hand figure and a very desolate theme with which the piece ends.
At least that is how it feels to me; it may conjure up quite different emotions in you. You might find the ending calm and comforting, for example.