Animations
Some things are easier to understand when you see them move: a value passing through a system, a tree rebalancing, anything that changes over time. For those, an explainer includes short animated scenes, drawn with shapes, arrows, labels, and graphs that build and move in front of you.
The animation is synced to the narration: the scene's steps play in the order the voice explains them, stretched to match the length of what's being said. It plays in place on the slide, like a short embedded video, except it was written for your specific question. For structure that doesn't move, the lesson uses a diagram instead.
Getting good animation slides
Most explainers include an animation on their own. If you know what you want to see move, say so: animate how the packet travels or show the array being sorted step by step.