In this video, the teacher explains the process of writing a thesis statement to express the main idea. They use the analogy of legs of a stool to represent the similarities and emphasize the importance of understanding what the prompt is asking. The teacher guides the viewers through the steps of identifying the main idea and expressing it in a response to the prompt. They highlight the significance of going beyond summarizing and instead allowing the audience to understand the thought process behind the main idea. By the end of the video, viewers have learned how to write a clear and effective thesis statement.