Viewport animation is similar in terms of settings compared to start and end timeline animations and scroll animations. Viewport animation is when an animation triggers on an element based on when the viewer scrolls into view of the set setting.
So when a viewer scrolls down to an area, an animation will trigger on an object. This is good for when you want to set certain animation based off of where the viewer is looking, instead of timeline which trigger based on the timeline settings, which wont work depending as it triggers as soon as the publication loads.
You can also use a setting for when the viewer scrolls out of view of the triggered animation and then back it will replay the animation, this is good to keep the consistency of animations along with keeping the viewer interested.