A trigger is an object on your PowerPoint slide - a picture, a shape, a button, or even a paragraph or text box. When you click on it an action is initiated. The action might be a sound, a movie, an animation, or text becoming visible on the slide.
"Here's a Power User column for teachers. Want to involve your students more in a presentation? Set up "triggers" for them to click as they go through the show. Triggers (related to animations) let you add surprise to your slides while inviting your viewer to take part and have fun."