An interior angle is formed when two sides of a polygon meet at a vertex. In simple terms, it is the angle inside a shape, such as a triangle or a square. The sum of the interior angles depends on the number of sides the polygon has. For example, a triangle has three sides and the sum of its interior angles is always 180 degrees. In a regular polygon, all interior angles are equal. The formula to calculate the sum of the interior angles of a polygon is (n - 2) \times 180 degrees, where n is the number of sides. For instance, a hexagon has six sides, and its interior angles sum up to 720 degrees.