1. Basic unit of the information is Step. It could be text, video, presentation slide or whatever Course's Authors decide. Step gives you Knowledge points, and you can see it in your Progress panel.
2. Several steps it's Lesson. Like the real lesson!
3. Several Lessons it's the Course Module, it's bigger unit of information, basically it's one topic or so.
4. And several Modules (or maybe only one), it's the Course.
* Also Courses give you valuable Certificates.