Incongruence comes from Carl Rogers' humanistic approach to psychology, and it reflects a difference between a person's self-image and actual experience. Almost everyone experiences a certain amount of incongruence in their life, according to Carl Rogers. It is rare for individuals to feel congruent in all areas of their life.
Rogers' approach to psychology suggests that humans want to experience and behave in ways that are consistent with their self-image and what they would like to be, their ideal-self. However, the further apart those two things are, the more incongruent an individual feels. When that happens, humans use defense mechanisms such as denial to feel less threatened.