Fragments (Animations)
Fragments are what you would typically call animations in PowerPoint. They allow you to show, hide and highlight different parts of your slide at a time. If you want to fade in an image when you step forward, it is a fragment you will want to use.
If you don't have any experience with Reveal.js's fragments, I suggest you start with the Basics of Fragments tutorial.
You can see a list of the available fragments here.