It's about a girl whose bf dumped her and got away with another girl. She couldn't see him anymore and once, they met. The boy's new gf probably asked him who she(the girl) was and the boy answered with a smile. Easy-peasy. But the girl was determined that she won't give away and let him let her down. Unfortunately, the boy broke off with that girl and came back to her. But she no longer believed him and told him to go away. That's why she said "goodbye to no one".
Hope my explanation helped. :)