Butterflies in the stomach is the physical sensation in humans of a "fluttery" feeling in the stomach, caused by a reduction of blood flow to the organ.
This is as a result of the release of adrenaline and norepinephrine in the fight-or-flight response, which causes increased heart rate and blood pressure, consequently sending more blood to the muscles.
[1] Butterflies in the stomach are usually linked in culture and language to the sentiment of love and romantic passion to a desired other.
One may feel butterflies in the stomach prior to meeting or confronting a love interest due to high levels of emotion and anxiety, as adrenaline and serotonin may be released when a love interest is concerned.
The symptom of this phenomenon is usually experienced prior to attempting to partake in something critical.