The lesson discusses the concept of captions, which are short descriptions or sentences that accompany images, such as in books, newspapers, or online articles. Students learn that a caption should be concise and clearly reflect the content of the image it describes. An example is provided where a child is cooking with their mother, emphasizing that the caption should convey the activity happening in the picture. Ultimately, the lesson aims to help students understand how to effectively write captions that enhance the understanding of images.