You are backstage and the guys don't know that you're there. You're watching them do their soundcheck.

The concert had already started, you were standing in the backstage, you heard the crowd screaming, and the screams were so loud you could barely hear your thoughts

The fans cheer as the boys start performing, they're singing "Dynamite" and having a good time. But the fans don't know that you're coming, you're backstage getting ready to go on stage.