Simply put Spiritual Development is about aligning the personality with the soul. It can also be said that Spiritual Development is about integrating our psychological self with our spiritual self.
Another way to define Spiritual Development is that by doing inner work you come to recognize that you have a self that has been conditioned by the world (the conditioned self/ego) and you also have a true self (your authentic self/soul) which you may have lost contact with.
Spiritual Development will reconnect you to your true self/soul or possibly you may be making conscious contact for the first time. The true self/soul has always been with you, it never left you but it may have been conditioned out of your awareness. Spiritual work is about coming back home to your true self/soul.