Imagine a line drawn through a shape. If the shape was folded along that line and the two halves fit over each other exactly, then the shape has line symmetry about the line.
Some shapes have only one line of symmetry, eg isosceles triangles.
An equilateral triangle has three lines of symmetry. A square has four lines of symmetry, although some figures have none and a circle has an infinite number. A rectangle is said to have rotational symmetry of order 2 . A square has rotational symmetry of order 4 . An interesting exercise is to write down the capital letters of the alphabet and try to identify the line and rotational symmetry within the letters.