Working at Google "unconscious bias" is a huge buzz word because we recognize it as a huge issue. I really love that we talk about it at least once a year company wide, and we also have to do mandatory "unconscious bias" training once a year too. So what is it?
Here's my own description, which is a little less politically correct than Google's description.
Unconscious bias: Generally speaking, we will always prefer people like us. We can be afraid of anything too different.
Why? Well familiarly breeds comfort. What race and religion are you, and what race and religion would you imagine was most "easy" for you to marry. It's the same one. You might be physically attracted to features of another race and intellectually curious about people who think differently to you - these differences can breed fasciation... but if you decided you didn't have the energy for that, you would take the path of least resistance and marry the girl who grew up next door.
If you discover someone grew up in the same town, you will probably light up. You have something in common - you have more shared territory. If you are a white male, and you meet another white male, you have that in common. Two people from the west or two people from the east are more likely to understand each other's jokes, values and way of thinking, so you're more likely to imagine that person as getting along well in the same team. Certainly you might recognize at times that some diversity is necessary, so that your group might think differently, but you will subconsciously also see that as a risk. They are less likely to be a culture fit.
At Google we train for unconscious bias based on the most obvious factors: (1) race (2) gender and (3) age. Let's not forget, however, that other elements of a person's background, like what they studied and their beliefs with religion and best work practices also come into play.
Obviously stereotypes play a huge role in this. And stereotypes are not without their value in humanity.... there is often some truth to them, and although people often break the mold, if we lack the energy to get to know someone, we might simply we apply the label we have from the stories we've been told, mixed in sometimes with our own experiences.
If I see a man with a gun I am safer to assume he is dangerous, instead of friendly. He might be friendly, I don't want that risk of talking to him though.
What we know of unconscious bias. We will always have it. No matter how many training sessions you have, it's best to just try and be educated and aware that it exists, so that you'll know when to ask a second opinion. Someone might get along great with you, but not with others. We see similarities in people and we might get excited.... shared experiences lead to this kind of positive bias, but we should also try to diversity out teams.