This really is one of those songs where you are supposed to put your own sentimental meaning to it. You could see it as him singing to a girl that he misses because she moved away, or even him singing to a girl who lives really far away. But my girlfriend passed away a little while ago, and then I came across this song. To me, it's him singing about someone he really loved, but she is gone. When he says "I'd send you a postcard dear, 'cause I wish you were here", I think he means that he wishes that he could send her a postcard, but he can't because she is dead. Also, he says "Oh if my voice could reach back through the past,.
I'd whisper in your ear". To me that means that he wants to reach back into the past when she was still there and tell her he loves her. You can think of it in a lot of ways, but that's how I see it.