Rolling Girl (ローリンガール) is a song sung by Hatsune Miku, and created by Genjitsutouhi-P.
There are many ways to look at Rolling Girl.
One of the ways to look at is that there is a lonely girl who is commonly bullied at school, and she always runs (or in this case, rolls) away from her problems.
Another way to look at is that there is a lonely girl who always needs help, but will never accept it from other people.
Yet another possibility is that the girl has a terminal illness, and finds herself shut out by her friends. Even though she is sad and in pain, she wants to continue living one day at a time. "I won't let this breath out," refers to her final breath, and at the end, she finally lets herself die.
The lyrics are vague, but the general idea is a lonely girl with far-off dreams and a history of "failing", and wants to "stop her breathing". At the end she is comforted by an unknown person.
Also, it can be viewed as a song about a girl stuck in an abusive relationship. The "dream" meaning that the guy will change, and with "no problem" abused people will say that what is happening to them is nothing, but it is always later made obvious that it is not true. The story for that would end with someone pulling her out of the relationship.
It also can be percieved that this girl is always expected to be perfect, but she can't, so she hurts herself (scratching herself), and runs away (rolls), hoping to get away from her problems. But in the end she is comforted, and told that she does not need to be perfect.
Another meaning that can be given to that song, is the story of a lonely girl who is tired of her life, as well as of the mistakes that she is making. That results with her starting to hurt herself by rolling. She also seems to push away from her a person who really cares about her. In the end though, she grows to be tired and ready to kill herself. And this is where the boy comes and tells her that it's enough, that she doesn't have to hurt herself anymore.
The video also helps in the meaning of understanding the song, since we can see that indeed Miku is pushing away the boy, by trying to strangle him. In the end after getting hurt, the boy comes to give her an end as she was wishing to. Strangle her back. However he grabs her from the hand and places her in his arms, telling her that he will help her, We'll hold our breath right now.
We have also to add the fact, that she is putting a facade in front of everyone with a smile, hiding her pain inside and hurting herself. The damage and pain that she is inflicting to herself is both mental and physical.
The final reason it could mean is about a girl that is constantly being abused and finally decides to end her life by comitting suicide, but an unknown boy jumps in and tries to save her, but in the end it is too late.
Another way to look at it is that she has VOICES IN HER HEAD, a mental illness. She just wants to roll/run away, however the boy wants to help her because in the end she starts self harming. When she gets hurt, he takes her in his arm and saves her, and she stops rolling.
This song hit #2 on the Weekly Vocaloid Ranking the week after it debuted in the Rankings. It's a generally popular song, and many people have done covers of it (one of the most common being Akiakane's cover).
It should also be mentioned that NicoNico singer, Akiakane, made the illustrations for the PV in MS Paint.