Implicit bias is attitudes or stereotypes that unconsciously affect our actions, decisions, and understanding. Since we are not really aware of when and how they occur, they can be difficult to challenge.
As we interact with others, we are taking in a ton of information- from what they look like, to how they are dressed, to the sound of their voice, and other environmental sounds. At the same time, we are also processing information from our body- are we hungry? Tired? Hot? Although we take in about 11 million bits per second of information, we are only able to process about 50 bits per second.
Implicit biases can be positive (like a preference for someone or something), but they can also be negative (an aversion of fear of someone or something). They are pervasive and develop over time. However, they are also changeable.
Here are some strategies to overcome implicit bias (taken from the US Department of Health and Human Services):

A great video example/explanation: https://www.youtube.com/watch?v=QnzkWc4b4mY