The glass cliff is like when you're hiking and there's a big cliff made of glass. It looks really pretty and shiny, but it's also very dangerous because it's so high up and the glass could break.
In the business world, sometimes companies choose women and other minority groups to lead when things are going really bad. They put them in charge when the company is already on the edge of that glass cliff, and the chances of failure are high.
This is not fair to those women because they are more likely to fail and might get blamed more than men leaders who would have been chosen for the job in a less risky situation. It's like asking someone to climb up that pretty glass cliff when you know they might slip and fall, instead of finding a safer route.
The glass cliff is an unfair situation for these leaders, who may be set up for failure just because of their gender or race.