The force in Dylan Thomas's poem is the natural energy that infuses and drives the universe. It pulses through the speaker, who is a natural being, just as it pulses through other parts of the universe. It is associated with vibrance and growth. The speaker writes that the same "green [young] fuse" that "drives the flower" drives his own youth. He likens, too, the natural force that drives water through rocks to the force that sends his blood coursing through his veins.
However, this same life-giving force is also a force that brings destruction and death. All that lives must die. Thomas's speaker notes that the same force that kills or "blasts" the trees will destroy him as well: the force that "ropes the blowing wind / Hauls my shroud sail," meaning that the life force brings him finally to death (the "shroud").
This force is a paradox, or seeming contradiction, because it brings both life and death. The same force that gives humans, plants, and other parts of nature the energy to grow and mature also ensures that the growth at some point will stop, and they will die. All of humanity—and the universe—is tied together by this natural force. Thomas ends the poem by saying,
And I am dumb to tell the lover’s tomb
How at my sheet goes the same crooked worm.
In other words, although he is unable to express to his lover's grave that he will end up in the same place, he understands that death will come to him too.