Is youtube claiming I don't own the videos I clearly do own? and what's going on? what are these "entities"?

I see this: "You have videos that may contain content that is owned by someone else. Please review these videos." on my youtube. I go to my videos and it says that the entities are Music Publishing Rights Collecting Society, and EMI Publishing. What are these things and why do they claim that they are a part of my videos? My videos are recorded by my mom's camera. They are videos of the Youth Symphony that I perform in. What do these "entities" have to do with anything? It says ads may be placed on my videos. So now they are making money off of my videos? someone please explain... -_-