A shape has line symmetry when one half of it is the mirror image of the other half. A mirror line helps us visualise line symmetry.

If a mirror is kept vertically passing through the centre of letters A, H, I, M and W, the exact same reflection appears.
If a mirror is kept horizontally passing through the centre of letters B, C, D, E and H, the exact same reflection appears.

Similarly, letters O and X have reflection symmetry about both horizontal and vertical mirrors.