If you're talking about the poem by Edith M. Thomas then I believe that the central idea is about how people can base something off of their looks. I'm not completely sure, but it talks a lot about how they look dead, but then explain that they are not. To me that makes it sound a lot like the saying "don't judge a book by its cover".
It could also mean that things take time to grow into something beautiful, and before that happens, you have to go through something difficult, seeming as if it is the end of the world. But then you blossom and bloom and everybody will look in awe.
I'm not completely sure these are right, and I'm not sure we read the same poem, but you didn't state the author's name. This was just off the top of my head but I hope it helps you or gives you an idea :)
