When you select something on a slide to be animated, you can choose whether you want a trigger. That will be either a click of the mouse or the Enter key. If you don't have a trigger you set a timer so the animation will happen after a certain amount of time or after a previous trigger.

Click on the Animations tab and you will see the menu bar. On the bottom of the menu bar from left to right you'll see Preview, Animation, Advanced Animation and Timing. In the Advanced Animation section, you'll see trigger options.