I think it is pretty self explanatory, but this song is is the whole "you don't know/realize what you've got till it's gone" thing.
"Only know you love her when you let her go. And you let her go."
Relationships are tough. You can start to feel as though things have gotten stagnant, or that you've lost feelings for the other person. Sometimes we think that the best action to take is to just 'get out'. However, it's only once you've left that person that you realize just how strong your feelings for them really are, how much you really miss them, how much you really love them.
You might realize this quickly, or you might not. The song is about living life with regret.