Beliefs are the stories we have about ourselves, others, and the world. They start to be formed when we are very young and are very pervasive in our day to day without us even knowing a lot of the time. They are formed during our upbringing, from the things we see, hear and experience every day.
Beliefs are not always the truth. They are your truth, based on your experience. They might be (and often are) formed based on biased and/or incomplete information.