Again this song is about his ex girlfriend Katie and how she cheated on Derek(lead singer).
He was away on a tour (I thing it was the one with All Time Low.)
and she met this guy Mike and she ended up cheating on Derek with him while he performed because he was never around. Mike reminded Katie of Derek in pretty much every way. And then coming home from a very stress full concert Katie finally fessed up. Though Derek had known all along.