The lesson discusses polygons, explaining that a polygon is a 2D shape with straight sides, such as a square, while a cube is not a polygon because it is 3D. Various types of polygons are introduced, including quadrilaterals like squares and trapeziums, as well as regular and irregular polygons, with specific examples like pentagons and hexagons. The importance of understanding the properties of both regular polygons (with equal sides and angles) and irregular ones (with unequal sides and angles) is emphasized.