Cognitive Bias is a Systematic error of thinking or rationality on judgement that influence our perception of the world and our decision making ability.
It is the mental shortcuts our brain makes to simplify deciosn making. These shortcuts can often lead to errors in judgment, irrational thinking or incorrect perception. These shortcuts are based on past experiences, emotions as we try to save time and effort due to our brains trying to process large amounts of information quicker.
As designers, by understanding what cognitive bias is and being aware of how it affects users, we can design interfaces that feel more intuitive, and easy for users to interact with, while also avoiding mistakes that frustrate or mislead the users. Avoiding cognitive bias is hard to do but staying aware of our own personal assumptions, preferences and experiences, can result in poor design, is important. To avoid this as much as possible it is improtant that we complete extensive user research early and often throughout the project. We also need to ensure we are getting diverse perspectives, base design designs on evidence and always challenge your assumptions.