What is meso compound?

A meso compound contains at least two stereogenic centers, yet the molecule itself is not chiral. This is because meso compounds contain an internal plane of symmetry; the molecule can be split by an imaginary mirror so that all atoms on one side of the mirror are the exact reflection of the atoms on the other side. This can be seen below for cis-1,2-dimethylcyclopentane; there are two chiral centers in the molecule since the two carbons labeled with the red asterisk are each bonded to four different groups. A mirror placed through the molecule, along the plane indicated by the dashed line, will exactly bisect the molecule with all groups exactly reflected by their counterparts on the other side of the "mirror".