The main theme of this poem is summed up in the opening line: "had we but world enough, and time... We don't have enough time or space, and the shortness of life, the need to live each moment while you can, is the poem' s theme. The language explores this with the images of vastness, slowness, leisure etc, contrasted with the rushing "chariot" of approaching death.
There are several resources on the poem, but I thought this essay was especially good.
There are several resources on the poem, but I thought this essay was especially good.