Symmetry is quite a common term used in day to day life. When we see certain figures with evenly balanced proportions, we say, “They are symmetrical”.
Suppose we could fold a picture in half such that the left and right halves match exactly then the picture is said to have line symmetry (Fig). We can see that the two halves are mirror images of each other. If we place a mirror on the fold then the image of one side of the picture will fall exactly on the other side of the picture. When it happens, the fold, which is the mirror line, is a line of symmetry (or an axis of symmetry) for the picture.

When we fold them along the dotted line, one half of the drawing would fit exactly over the other half.