If you have a course with a lot of lessons, it can be easy for students to lose track of where they are up to, especially if they’re taking the course over a period of several weeks or months.
Sensei Course Progress provides a sidebar widget that displays all the lessons in the current course (or module, if you’re using the Sensei Modules extension), and lets your learners see, at a glance, which ones they have completed and which they have yet to take.