That is Darwin theory of evolution as a whole with other rules applied to it that theory however could by today's standard be only apply-able to animals in the Wild therefore the theory is more like 'evolution of animals in the wild' , but the general theory of evolution is the changing of the body to meet the demands of the environment you're born in, for example someone who lives in Desert conditions would have much less body hair then someone living in Antarctic conditions.
These days however we live in a easy environment which hardly has any struggle to live when compared to animals in the wild, as a result our body is weakening because our bodies don't struggle to do anything which needs improving and our bodies are weakening as the environment we live in doesn't need extreme strength to live in so we a weakening down to the strength we actually need to survive.
Evolve more or less means the changing of the human body and not necessarily our body improving to be better than the bodies of the previous generation, for example
if strength is what the previous generation wants to be improved as they reached the end of how high their strength can get the theory is the next generation (assuming both parents did what the previous generation did) strength will reach a greater end than the previous generation strength (evolution).
However if the previous generation did want the strength to be improved but decided to be lazy and not reach anywhere near the end of their full strength the next generation wouldn't have be any better which the strength at all and in theory the strength would remain the same (no evolution) or get weaker (evolution).
That is the actual understanding of evolution, however devolving is impossible to do, why you ask? Simple you cannot de-age a person and make them go from 10 to 5 (at the moment) and time cannot be reversed at all so in all devolving is actually impossible because humans can evolve to match similar strength and body of a being back in the chain, but the thing that bring us forward in the chain is one this and that is only one thing... TIME.