Powtoon is an online tool to create animations in an easy way. The tool offers a lot of templates that make it quick to set up. A lot of them are free to use, but some require an upgrade.
Creating an animation could make difficult concepts more understandable for students. By making it available online, students can also rewatch it when they want to.
Example of an animation explaining a concept:
With a free account you can create animations with a duration up to five minutes.