What are optical isomers?

I mention hands because they are a manifestation of chirality, of handedness, that is easy to appreciate. Your left hand and your right hand are (to a 1st approx.) identical, and yet they cannot be superimposed upon each other. Nor could your right hand be superimposed on the left hand of your identical twin.

An optical isomer is a species that CANNOT be superimposed upon its mirror image. Of course, the connectivity of each isomer is still the same. And yet, their handedness, their chirality, entails a physical difference.

In most instances, the optical isomers will have precisely the same chemistry. Introduce plane polarized light, however, and each isomer will have a different interaction: one isomer will rotate the light in the opposite direction to the other. Introduce another chiral molecule, and each isomer will potentially have a different chemical interaction.

In practice, we are first exposed to handedness in organic chemistry. Any carbon that has 4 different groups attached to it, i.e. #CR_1R_2R_3R_4#, is in principle chiral, and can generate a pair of optical isomers depending on the orientation of the #R# groups around the tetrahedron.

So the succinct answer to your question: an optical isomer is a molecule that cannot be superimposed upon its mirror image .