Butterflies in the stomach is a medical condition characterized by the physical sensation of a "fluttery" (hence butterflies) feeling in the stomach. Some believe that this is caused by the release of epinephrine, or adrenaline, when one is nervous, pulling blood away from the stomach and sending it to the muscles. Butterflies in the stomach is most often experienced prior to important events, when stress is induced, but can be experienced in situations of impending danger.
The phrase 'Butterflies in the stomach' is an example of an idiom.