Dissonance is the tension that occurs in a subject when they perceive that something is wrong within them, or within the world.
‘Wrong’ here means something like ‘against the held world-view,’ so that dissonance is the tension due to a broken world-view.
To make the concept clearer, consider an example:
Sarah thinks of herself as fit and athletic. She was on a sports team in high-school and has maintained this view of herself. She decides to go for a run, and upon realizing that she runs out of breath quickly, senses that something is wrong. She understood that she was athletic, but now she is presented with evidence that she is not. This tension between her beliefs that she is athletic and that she is not athletic is dissonance, and causes her suffering.