Interior angles are the angles formed inside a polygon when two adjacent sides meet. In a triangle, the sum of the interior angles is always 180 degrees. For other polygons, the sum can be calculated using the formula: (n - 2) × 180, where n is the number of sides. For example, a quadrilateral has four sides, so its interior angles add up to (4 - 2) × 180 = 360 degrees. Each interior angle can vary in size depending on the shape of the polygon, but they all contribute to the total sum based on the number of sides.