An exterior angle is formed when a side of a polygon is extended, creating an angle outside the shape. In a triangle, for example, each exterior angle is equal to the sum of the two opposite interior angles. This relationship helps in solving various geometric problems. Exterior angles are important in geometry because they provide insights into the properties of polygons. The sum of the exterior angles of any polygon, regardless of the number of sides, is always 360 degrees. This rule applies to triangles, quadrilaterals, and other polygons, making it a fundamental concept in geometry.