For each individual, we have unique experiences that shape who we are. It is our life experiences that allow people to develop their core beliefs.
Core beliefs are the thoughts a person has that determine how they interpret their experiences. For example, if someone has the core belief that "The world is a bad place", they will believe that people who are kind have an ulterior motive (someone being kind without reason just doesn't line up with their world view).
Challenging these negative core beliefs can help develop a healthier understanding of ourselves and the world around us.