Although the word "dao" can have multiple meanings, at its most basic level it means "the way."
"The Way" is simply the natural flow of life, like water.
Water always effortlessly flows downwards (never upwards).
So too a true understanding of the natural way of things means not having to exert force or energy because one is just going with the flow of nature.
One doesn't have to try, because one is already flowing in the natural direction of the world. This is the basis of Daoist philosophy.