Imagine this situation, you are at school and someone you know comes by, you say hello, and this person just gives you a quick, unfriendly “hello” and then walks away. How would you attribute this situation — why did this person act this way If you react to this situation by saying the person is a “jerk” or an “ass”, then you have made the fundamental attribution error; the tendency for an observer, when interpreting and explaining the behavior of another person (the actor), to underestimate the situation and to overestimate the personal disposition.
Maybe the person was having the worst day of their life, just found out a loved one died, failed a test and was feeling devastated, etc. In this case, the situation may have caused them to act in a way that was different than their normal happy self. But, you, as a normal observer, would instead attribute their behavior to them as a person…acted that way because that is the type of person they are.