Lesson 15 focuses on understanding and creating a rainfall graph, which is visually represented as a bar graph to depict the average monthly rainfall. The example provided illustrates the rainfall patterns in the Maltese Islands, highlighting January, December, and October as the wettest months, and June and July as the driest. The lesson emphasizes the importance of using appropriate scales and labels when drawing the graph, ensuring accurate representation of data provided by meteorologists using instruments like the rain gauge.