What does reflectional symmetry mean in math?

Answer 1

It means literally if you reflected the object through the centre of the image, at a specific angle, in a mirror you would get the same image on the mirror with that on the other side.

So or example the letter A has one line of reflectional symmetry because if you put a mirror vertically along it, you would see the same image on the mirror as the side that you weren't looking at.