When the world is in possession of the Tao,
The galloping horses are led to fertilize the fields with their droppings.
When the world has become Taoless,
War horses breed themselves on the suburbs.
There is no calamity like not knowing what is enough.
There is no evil like covetousness.
Only he who knows what is enough will always have enough.
This is a very straight-forward passage. Essentially, when you are living in balance and contentment with what you have, you are in a state of peace and oneness with the world. But when you are striving for more, and attempting to grasp and hold material things, then you are at odds with the world. If you believe that you are lacking, then you will never be content with what you have. This feeling of not having enough is what leads to war, societal problems, anxiety and depression, a veritable smorgasbord of social ills.
Ask yourself: Do I really need the things I think I need?