It is an imaginary line which divides the image into two equal halves. It could be horizontal, vertical or diagonal. There could be one or more than one line of symmetry in a figure.
If all the sides and angles of a polygon are equal then it is said to be a regular polygon. Like the equilateral triangle, square etc.
All the regular polygons are symmetrical shapes.
In the regular polygon, the number of lines of symmetry is the same as the number of its sides.