"She's still here, honey. People don't ever leave us for good." To me, that quote means that after people die, their spirits are still alive in the hearts and minds of all the people that love them. It may also mean that people will reunite in the after life.

The quote is important to the theme of the book, which I think is that even if something horrible, like death, happens in your life, you must stay positive and keep on living for your loved ones that are still alive. It's what your dead loved one(s) would want you to go on living a happy life while you can.

At one point in the book, Summer thought Uncle Ob was so miserable that he would just let himself die. I think because Ob believed May was still with him and because Summer and Cletus were there, it kept his faith alive and made his will to live stronger.