The lesson module presents a series of HTML pages to the student who is usually asked to make some sort of choice within the lesson. Unlike a book, a lesson can include scored questions (multiple choice, essay, short answer, true/false). The lesson activity allows for multiple choices and paths based on those choices.
Lessons work well when you have content (text, videos, images) and you also want to assess student engagement or knowledge during the course of the lesson. A constraint of the lesson is that it's not incredibly robust for scoring. It likes to mark the entire lesson as correct or incorrect (i.e., 10 points or 0 points, nothing in the middle).
Note: A Quiz activity is more robust and can do the same thing.