Hi Alain,
In Moodle-speak, students are enrolled in Courses. Inside Courses, teachers can provide various Activities (Forums, Quizzes, Lessons... - as Mary explains, they're available in a dropdown menu) and Resources (PDF documents, videos, audio recordings, links to Internet pages...)
On the Moodle page, the Course can be subdivided into Sections (Topics or Weeks). Each Section can have a number of Activities and Resources.
Maybe Section corresponds to what you think of as a lesson.
Here is an example of a 15-week course in which each Section represents one week's work for the students. Sorry, no Lessons (Moodle sense) - I prefer to use QuizPort.
Cheers,
Glenys