Cognitive dissonance is when two or more thoughts or feelings you have are in conflict, or a thought/feeling and an action. Our brains like consistency, and our egos especially like self-consistency, so this conflict feels unpleasant. Its like if you pointed out something someone did wrong, and someone said "well you do that too". All of a sudden your brain is like "shit, i think doing that is wrong but i did it", and that feels uncomfortable. Often times the solution is to justify the conflict by pretending there isnt one, for example by saying "its okay when i do it because..."