Most of us, despite our explicit beliefs, have implicit biases. These are assumptions we make about social groups that inform our language, explicit attitudes, and actions in subtle ways. Implicit biases can be so deeply engrained that they inform our interactions with others when we aren't even consciously aware of them. What this means is that even the most egalitarian-minded among us may unwittingly act in ways (or hold beliefs) that are racist or sexist, despite their desire not to do so.
Stereotype threat happens when a person's group identity, shrouded by the dominant culture's stereotypical judgements and expectations, negatively affects her own capacity to succeed. The fear, conscious or unconscious, that she will be judged according to negative stereotypes can raise self-doubt and lead a person to effectively impair her own performance, thereby narrowing her educational and career prospects.
There is a great deal of evidence to show that implicit bias and stereotype threat contribute to both to the lack of diversity in philosophy and to the perception of philosophy by women and other marginalized groups as a white male discipline.
We believe it is essential to raise awareness about biases and stereotypical thinking. We also believe there are practical measures we can take to address these problems, from identifying and eliminating biases and stereotypical assumptions to effecting practices that limit their influence on our professional and social interactions with others.
For some further reading, see the Implicit Bias and Philosophy International Project