A molecule containing exactly one asymmetric carbon atom is completely asymmetric, thus it does not contain a S_n and is chiral, e.g. lactic acid.

mirror image lactic acid

If a second asymmetric carbon atom, constitutionally identical to the first, is added, then it is possible (not necessary, depending on both atoms' configurations) that a symmetry is newly formed and chirality is lost.

If this is the case then we have a meso compound. One carbon is of R configuration, the other of S configuration, they compensate intramolecularly, and the molecule of a meso compound is achiral containing a S_n.

Example meso-tartaric acid HOOC-CH(OH)-CH(OH)-COOH:

mirror image meso-tartaric acid syn-periplanar

mirror image meso-tartaric acid anti-periplanar

mirror image meso-tartaric acid gauche

Convince yourself that the mirror images are identical to the originals, i.e. that there exists one meso-tartaric acid only.