In Hopkin's poem "God's Grandeur" the first line of the second stanza states, "And for all this, nature is never spent". The meaning of the word spent in context of this line means: worn out.
Therefore, the power of nature, given God's power, is unlimited. Nature, according to Hopkins, has the ability to regenerate itself over and over again. The world "will flame out" and "gather to a greatness". Nature, unlike the generations of men who "have trod, have trod, have trod", is able to shine again at morning even though the world sees nothing but darkness at night.
Basically what Hopkins is saying is that regardless of what man does, no matter how hard he toils, he is unlike nature- he is mortal in a sense. Nature, because of God, is immortal- able to survive the footprint of mankind's generations.