In a behind the scenes video, Sam says that this song is about what happens to a couple after they breakup from a relationship but here's how I see the song. Keep in mind, you should see the music video first so you'll understand about what I'm talking about. It starts off with Sam singing about someone ready to leave or stop the relationship. The line "What's a goodbye good for?" touches me quite a bit as it really is true in real life. What is a good bye good for? It should be used as a see you later sort of saying this couple is getting ready to leave each other forever. They 're very rarely gonna see each other again if at all. Into the hook now, "I'm trying hard to forget you, but my empty walls won't let me let you go," means that the other person might not be there but the memory of relationship will be which ties into my next point. The shadow is the relationship they started. The memory of that won't leave either of them. Verse 2 is about one person in the relationship forgetting about the other person really easily while the other person is having a hard time forgetting the "shadow." Now the bridge. After all the forgetting, the first person who broke up can't forget what they started. The other person meant too much for them and the shadow came back as a result of the breakup. At the end of the song, it ends with the first line. This means that one person blamed the other person for the result of the breakup. The music video fits really well with the lyrics since at one part the male shadow dancer holds hands with the female dancer really embracing the line "You can leave me, Can you leave me alone~?" although it come a minute before the line. Lastly, the link the music video. https://www.youtube.com/watch?v=CEQvj0qszlw.