A shape has line symmetry if it can be folded along a line so both halves match exactly. Some shapes have multiple lines of symmetry. Squares have 4, rectangles have 2, circles have infinite.
You already know what a line of symmetry is: it's the fold line that cuts a shape into two mirror-image halves. Now the question is: how many such lines can a shape have? The answer depends entirely on the shape's structure, and exploring different shapes reveals some satisfying patterns.
Start with the simplest case: a rectangle that is not a square. You can fold it top-to-bottom (a horizontal line through the middle) or left-to-right (a vertical line through the middle), and both halves will match. But try folding it corner-to-corner diagonally — the halves won't match because the rectangle is longer in one direction than the other. So a rectangle has exactly 2 lines of symmetry.
A square is different. Because all four sides are equal, the diagonal fold works too: folding corner-to-corner produces two matching right triangles. A square has 4 lines of symmetry: horizontal, vertical, and both diagonals. The more equal and regular a shape is, the more lines of symmetry it tends to have.
At the extreme is the circle, which has infinite lines of symmetry — any diameter divides it into two identical halves. This is because every point on a circle is exactly the same distance from the center, so any fold through the center works. Some shapes, like a scalene triangle (all sides different), have zero lines of symmetry. The number of lines of symmetry is a property that captures something deep about a shape's regularity: the more symmetrical a shape, the more ways it can be folded onto itself.