Thomas Hardy was a British novelist and poet writing during the late nineteenth and early twentieth centuries. He's primarily remembered for his novels, like Tess of the D'Urbervilles and Jude the Obscure, but he started and ended his writing career as a poet. In fact, the scathing reviews in response to Jude the Obscure in 1895 drove him to give up writing novels altogether – after that, he switched back to poetry and never wrote another novel, though he lived until 1928.
"Afterwards" was published as the grand finale in a volume of poems entitled Moments of Vision, which came out in 1917. The poem was written towards the end of Hardy's life, and it reflects the bittersweet (more bitter than sweet) nostalgia of an old man looking back over his life and realizing that he's got fewer days ahead than he has behind. It was also written during World War I, and reflects some of the despair and anguish typical of British literature written during that time – no one had ever seen death or destruction on that scale before. So if "Afterwards" seems kind of dark or depressing to you, you're not alone: Hardy certainly felt that he had plenty to be depressed about.
In "Afterwards," Thomas Hardy describes a lot of natural images, like "hedgehogs" and "dewfall-hawks." If you're not a fan of rodents or birds of prey, however, don't assume that this poem has nothing to offer you: it's about much bigger ideas than the "glad green fields" that the speaker describes. It's about the fragility of life and the certainty of death. Pretty heavy stuff, we know.
But it's not just a meditation on mortality; it's about the anxiety that the speaker feels about the world continuing to turn after he has died. The poet imagines what his friends and neighbors will say about him after he's dead. Even if you don't think about dying as often as the poet – Hardy was, after all, 77 years old when he wrote this poem – you can probably still sympathize with its basic idea. Have you ever moved to a new place, and thought about how your old friends and neighbors would remember you? Or realized that your friends would continue on with their lives, even though you weren't there? So, sure, you can read it for the beauty of the imagery, but you can also think of it as a poem about what we leave behind: in other words, it's about what happens "afterwards."