The line of symmetry is a line which divides the shape in two equal similar parts. It is line along which if shape is folded then two shapes overlap each other and both are exactly equal to each other.
For example, the shape of square. If the square is cut or fold along it diagonal then two similar traingles are obtained that are exactly simillar. So diagonal of a square is a line of symmetry for the square. There is possible that a figure has more than one line of symmetry.
If any line does not divides the shape in two equal half, then such line is not a line of symmetry for that shape.