Both of these molecules are 1-Bromo-1-chloroethane. But they are not exactly the same molecule, in the same way, that your left shoe is not exactly the same as your right. They are non-superimposable mirror images of each other. How do we communicate this difference?
One way would be to describe their physical properties. For example, although these two molecules have the same boiling point, melting point, and share many other physical properties, they rotate plane-polarized light in equal and opposite directions, a property called optical rotation. We could use (+)-1-bromo-1-chloroethane to refer to the isomer that rotates polarized light to the right (clockwise, or “dextrorotatory”) and use (-)-1-Bromo-1-chloroethane to refer to the isomer that rotates polarized light to the left (counterclockwise, or “levorotatory”).
However, this nomenclature suffers from a serious problem. There is no simple correlation between the arrangement of substituents around a chiral center and the direction in which polarized light is rotated. Another solution is needed.