What is the "glass ceiling" in the professional world of western countries?

The term "Glass Ceiling" is a saying/term for the commonly shared idea within Western Countries that an invisible barrier exists which keeps an given existing group (in most cases, the group is women) from promoting beyond a certain point within any job or corporation industry.

When someone says, "She was unable to become CEO because of the glass ceiling", they are saying that due to either certain rules set in place or attitudes from a more likely-than-not male superior supervisor, the woman will be unable despite her best effort to "reach past the ceiling" into what is above i.e higher/est position the job can offer.