Surely not the best known neither most played from Vierne's pieces, but IMHO at least equal to the better known "Clair de lune" from op.53.
It is a kind of very singing Nocturne (in d flat, the typical tonality for nocturnes), with a cello-like theme. The sky becomes suddenly darker and threatening in the agitated central part ; the calm comes back for the peaceful last part (flute harmonique and voix céleste).