The title, "The Wild Swans at Coole," refers both to swans in the wild and the place where they enjoying being wild: Coole Park, in Ireland. The poem itself describes a number of natural features and is about both the beauty of nature and about the energy that characterizes the natural world. Indeed, for the speaker of the poem, nature is both powerful and beautiful, something that appears not to age and change the way the rest of the world starts to droop and sag. While the speaker implies at the end that the swans will fly away, for the time being they represent something that is static and "still"—a sad, yet dynamic, reminder of what he'll never be again. Bummer.
For the speaker of the poem, nature is unnatural. That is, it appears not to age ("their hearts have not grown old") according to a natural cycle. Weird, but powerful.
Although nature's tranquility and seeming changelessness is enviable (the speaker wishes he could possess those qualities), it is also kind of scary. Without change, there can be no learning. The speaker should be thankful for old age, since it has allowed him to craft such a doggone powerfully reflective poem.