Gerard Manley Hopkins, in this poem, is praising the continual rebirth of nature and the fact that it displays God's creation and His "dearest freshness." In stating that nature is never "spent," he means, essentially, that it can never run out of this quality which makes it what it is, namely, God's grandeur. Nature does not flower and then run out of flowers. On the contrary, the flowers that die in winter will return in the spring, much as God's grandeur remains on the earth in perpetuity.

Hopkins explains that the Holy Ghost "broods" over the world eternally, his "warm breast" and "bright wings" ensuring that nature will never be "spent." Despite the fact that generations have toiled upon the earth, tilling the soil, treading it down, and often failing to recognize the glory and power of God, nature itself remains unconquered and is sustained by the presence of that Holy Ghost which created it in the first place and which continues to give it strength and succor.

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.