When we are communicating as humans, we have certain expectations about how people are going to respond. For example, when you ask the question "How are you?" you would likely expect someone to respond, "Good, how are you?" Expectancy violation theory is the theory that responses that go against our expectations are preferable as humans. So, if someone responds in a way that you are not expecting, you might be surprised and, therefore, it might capture your attention more than if someone responds in a typical way that you were anticipating. This would be an example of expectancy violation theory at work.
Violation valence is a component of expectancy violation theory. It has to do with behavior (nonverbal) actions. When someone behaves in a way that you do not expect, you will either respond positively or negatively to this action. This reaction is called violation valence, and refers to your emotional response to the unexpected behavior.