The 'not completed' indicator is a useful tool to see how engaged your students were with your lesson, and the meaning is situational depending on where they interacted with your content.
When looking through the results of a lesson shared in a module with your students, 'not completed' means that a student has not viewed the slide through the lesson overview in their LessonUp environment, via the student app (www.lessonup.app).
For a lesson taught in class, 'not completed' indicates that a student did not have LessonUp open during that section of the lesson. This could be because they had another tab or program open instead!