It's the idea that we develop spiritually over time - so much, in fact, that the development can be called "evolution" in the same way that we have developed so much physically over time and we call that "evolution". In New Age, there is a theory that our physical and mental evolution has caused us to devolve spiritually and so we must find a way to evolve spiritually again. For example, atheism shows a major lack of spirituality but a height of mental development while monotheistic religions developed people spiritually (and physically) but left little room for mental development. As society changes, some things become lacking - even physicality is lacking in many people that seek to develop mentally and spiritually. If we do not pay attention to all forms of evolution, some forms may "devolve" - that is, we won't lose those things, but they will get stuck, frozen, and it'll be harder for us to develop them again.