This song is about a girl who left during the summer, not realizing the reality of things, and who boyfriends will cheat, which is what happened. Her boyfriend found anew girl, which happened to be her best friend. At the end of the song, both friends realize what time they wasted on that guy, and he gets neither of them.
For those of you who think that Hey Monday is trying to copy Paramore, I think you are wrong. Both bands are amazing, and have their own styles. They sound nothing alike. I truly do not understand what joy it brings to take the time to look up songs of bands you don't like and post rude comments about them.