I think "boo" is like a pet name, the equivalent of shnookums or wumbly-bear :) So the song is basically saying that even though the two people singing aren't together any more they will always mean to each other what they meant before, and they'll always think of each other in terms of their old petnames.
I think "boo" is like a pet name, the equivalent of shnookums or wumbly-bear :) So the song is basically saying that even though the two people singing aren't together any more they will always mean to each other what they meant before, and they'll always think of each other in terms of their old petnames.