The girl cheated on the guy last Summer, but the girl feels really bad about it, but won't tell him, yet he knows. They know their relationship is ending, yet nether can seem to let go. If you listen to the song, when Shawn is asking where she's been, etc, Camila is trying to just say "Hold me close" this doesn't have to end, don't say anything, don't talk, don't try to fight with me, "Hold me close."
Throughout the whole song, Shawn is asking where Camila has been, Camila is trying to avoid it. "I know what you did last Summer,"