If life can grow longer than fireflies, where does tobacco ash come from? What do you mean?

It is difficult to send books from Yuyang Road, and Hengyue Mountain is not in the high moon. Xuan Yan has feelings and embroiders, but Gui Ling has no desire for a golden cup. If you live a long life, fireflies will turn tobacco into ashes. Supplement: The whole meaning of poetry is a kind of missing that can't get together ... The first sentence, I was awakened by thunder in my dream, which has been remembered for many years; The second sentence, letters can't be posted because of the long distance, and the tall Hengshan Mountain also blocks the moonlight (there is not a poem called, the sea rises and the moon rises, and the sky is still close, here, the poet doesn't even have a chance to send the thoughts of the moon); The third sentence probably means that the swallow has feelings (the swallow came back in spring, so it is said that the swallow has feelings), but the turtle did not respond. My understanding is that he is divining the good and bad luck of leaving people, but he can't get an accurate divination, so Jinbei is not clear ... The fourth sentence roughly means that if he can keep his appearance private with you, he must wait until the fireflies spontaneously ignite and the grass turns gray ... or it can be understood this way.