A line of symmetry is an imaginary line that divides a shape into two identical halves. When a shape is folded along this line, both sides match perfectly. For example, a circle has an infinite number of lines of symmetry, while a rectangle has two. Shapes can have different types of symmetry. Regular polygons, like a square, have multiple lines of symmetry, while irregular shapes may have none. Understanding lines of symmetry helps in various fields, including art, design, and mathematics, as it reveals balance and proportion in objects.