I went to school and learned a lesson. We often hope that our children learn from these lessons, but sometimes the lesson learned was not the one intended.
This could be called a useful piece of practical wisdom, and for me that was, don’t get into fights at the back of the schoolyard.
The former lesson can be, on one hand, a section of school work, from a larger continuous topic, or, part of a book, which can be an exercise.
Then there’s the study of the past and the hope that we can learn from the lessons of the past.
Sadly, in a lot of cases, we don’t and are therefore doomed to repeat the past, only with far more devastating consequences.
A lesson can also be a passage from the bible.
Or is it lessen, where we reduce the costs.
So lessen means reduce, to make less.