The lesson focuses on understanding trigonometric concepts such as sine, cosine, and tangent, particularly how to apply them in solving problems involving the adjacent and hypotenuse sides of right-angled triangles. The instructor emphasizes the importance of balancing both sides of equations and using diagrams effectively to visualize trigonometric ratios like (cos(39^circ)), which involves solving for the hypotenuse in a given context. The lesson includes practical applications and encourages aligning concepts with diagrams and equations to resolve mathematical challenges accurately.