What is it?
Belief is any one of the many ideas in our mind that we hold to be true.
Why is it important?
Belief is rather like our internal mental map of reality. We have beliefs about basically everything we "know" - some of our beliefs we test while some we do not allow to be challenged. Some of our beliefs cannot be proven true or false and are ideas we accept on "faith". We think, speak and act according to our beliefs - using them as the internal reference points for our perceived and accepted reality. As we make new discoveries in life, our beliefs may change, and this is the essence of learning new things. People unable or unwilling to let go of obsolete beliefs - those that are proven untrue - are unable to enjoy the benefit of new learning and persist in using "faulty maps" to navigate reality. Figuratively speaking, some people insist the Earth is flat - which may negatively impact their ability to successfully navigate our world.