Margaret, are you grievingOver Goldengrove unleaving? (1-2)
The speaker uses the made-up word "unleaving" when addressing the child, Margaret. It sounds like the kind of word a child might invent to describe something like the leaves falling off of trees.
Now no matter, child, the name:Sorrow's springs are the same. (10-11)
The speaker tells Margaret not to worry about "nam[ing]," or labeling, her sorrow. Communicating it isn't really important because, really, everyone has the same sources or "springs" of sadness. It sounds like he's comforting her. Maybe she seemed frustrated by her own inability to explain why she was so sad. Have you ever seen a kid who gets even more upset just because she or he can't explain what's up?
Nor mouth had, no nor mind, expressedWhat heart heard of, ghost guessed: (12-13)
The speaker explains that although Margaret can't explain her sadness either verbally (with her "mouth") or even in her own thoughts (in her "mind"), her "heart" still understands it, and so does her "ghost," or spirit. Try using that excuse next time you have trouble expressing your ideas in an essay!