Stereotpe Threat is the observance of a performance decrease when the person is aware of negative stereotypes towards one or more aspects of their identities. That's confusing to say and write. Better explained: when you're aware that some aspect of yourself (race, gender, age) is negatively associated with performance, your actual performance is impacted negatively. When you're less aware of the stereotype, the observed impact is lessened.
This is a familiar occasion in my life and many others. From being the only female developer on an all-male team, to being plus-sized, to appearing much younger than I am, I can quickly call to mind times when I was acutely aware of my "failing" and that it was to blame for the discrepancy in performance. For example, when I have to have difficult conversations with another person (asking for a raise, for example), I have often thought to myself that they weren't going to take me seriously because I'm young and female. Any emotion I showed felt it would be chalked up as "She's a girl, she's just being irritable and emotional." This causes so much frustration inside of me that I do get vexed and actually start to cry. It's completely out of my control, which makes me even more frustrated, and I become so overwhelmed feeling the stereotype that I become the stereotype. I never got a raise, I was passed over for promotions.
As I've gotten older, I've gotten better at pushing aside that idea in my head at the start and instead picturing myself staying calm and collected. This does actually help confrontations go better, and I feel more confident because of it. Criticism is also easier to take constructively when I decouple the feedback from the act of being a female. Once I'm not feeling guilty just for being me, I'm able to focus on the challenge in front of me.