It was a form of protest. We are blind to so many injustices in the world especially to the injustices that happen to women. I read this in one of her interviews. It was a performance piece. She also crocheted while doing this.
The subject of a sentence is the who or what doing or being something. "Yoko" is the subject; "invited" is the verb (Yoko is doing something). Another example: Yoko is a wonderful host. "Yoko" is the subject, and "is" is the verb (Yoko is being something).