The affect heuristic is a decision making shortcut whereby people will base a decision on emotion rather than logic, especially in the case of complex problems that may be more conveniently explained (typically) with a stereotype. Essentially, the affect heuristic explains how our mind takes a difficult question and subconsciously substitutes it for an easier one.
An example of the affect heuristic in action would be a motorist experiencing road rage after being cut off by another motorist. Although this person has been held up by only a matter of seconds they are more likely to react emotionally than logically and can become intensely angry at the other motorist. They may, in fact, choose to do something completely illogical and go out their way to confront the other driver, costing them even more time than they were angry about losing in the first time only to make a point.