This lecture covers the concept of changing variables in a double integral, where two open bounded subsets D and E of R² are related by a bijection F: E → D. The instructor explains how to describe F, define the Jacobian matrix, and apply the change of variables formula. Through examples, the lecture illustrates how to transform double integrals using different variable changes, emphasizing the importance of the Jacobian determinant. The lecture concludes by showcasing the practical application of variable changes in double integrals, providing a comprehensive understanding of the topic.