The theory that emotions are interpreted based on physiological reactions to events (later replaced by Cannon-Bard theory, which states the opposite).
I think this little example from http://changingminds.org/explanations/theories/james_lange_emotion.htm helps immensely: Event ==> arousal ==> interpretation ==> emotionThat link can also provide you with several other examples and more information on this theory.
Example: While walking in the woods, you stumble upon a bear. Your heart begins to race and you notice that you are trembling. Thus, you conclude, you must be frightened.
Fournier, G. (2010). James-Lange Theory. Psych Central. Retrieved on April 25, 2015, from http://psychcentral.com/encyclopedia/2009/james-lange-theory/