In the context of this lesson, what is plot?

We cannot really answer for sure, given that we do not know what lesson you are talking about. But generally, plot is what the story is about, what is happening inside a story. So when you read the lesson, you will know what it is about, and that will be its plot. For example, the plot of the Harry Potter novels is about a young wizard Harry who has to defeat his mortal enemy, Voldemort.