The idea of gender as a performance, refers to the daily application of beliefs that were learned and applied, based on cultural norms of what gender is. Norms of what femininity and masculinity look like. I think what Butler is getting at, is that gender is not completely natural. It is a performance or application of beliefs that become normalized over a period of time. It has to do with what we think validates being a man or being a woman. An example would be something like, being a man or woman has to do with the type of clothes you wear or type of haircut you have.