A subreality is a collection of beliefs, some true and some false, that support each other to form a logically consistent thought model. It is impossible for a human being to have a full knowledge of everything since we have limited senses and intelligence, but we can try to have as many as possible of our beliefs represent the reality by searching for the truth and the use of logic.
This is something many don't want to or are incapable to do. Living in a static subreality is more comfortable than keeping the beliefs open for questioning and ranked by propabilities, the core beliefs with the highest propability forming the core of the beliefs system. People suffer from cognitive dissonance when their solid beliefs are constested. It is unpleasant and many reject all conflicting information and prefer to live in a blisfull ignorance.