You probably noticed that this poem's title and the first line are identical. If you didn't, well, you really need to work on your close-reading skills.
Thomas didn't title this poem (and quite a few others as well). It is a convention to use a poem's first line as the title when the poet doesn't include one. We need to call it something, right? If we just said, "you know, that poem by Dylan Thomas about death," that could lead to lots of confusion.
At the same time, that first line makes an apporpriate title because it does a good job of introducing the major ideas to follow. First up, we're introduced to "the force" (time), which of course is the major preoccupation of the poem's speaker. Secondly, we get "that through the green fuse drives," which underscores the driving energy and vitality of life that is also a big component of the poem. And finally, we finish with "the flower," a pretty stock symbol of the natural world that the poem also focuses on in the lines to come. To sum up: we get time, energy, and nature right from the jump—ideas that will continue to be focal points throughout the poem. As a first line, we'd say this makes a pretty effective title, right?