What are triggers?

Trigger: Shift lever with a "push" function. Pressing the shorter lever usually shifts to a higher gear on the rear derailleur and to a lighter gear on the front derailleur.

Triggers:
 Shift lever with "push function". Pushing the shorter lever normally shifts to a higher gear on the rear derailleur and to a lighter gear on the front derailleur. Similarly, the longer lever shifts to a lower gear on the rear derailleur and to a higher gear on the front derailleur.

Relevant articles:
Adjusting the bicycle gears