A Meso compound or meso isomer is a chemical compound with molecules that contain 2 or more asymmetric (chiral) centers but which is optically inactive (or achiral) because it contains an internal plane of symmetry. For example one of the isomers of tartaric acid is a meso compound:

If you rotate the molecule 180° you will have the same apparent stereochemistry. (The bottom must be rotated within the plane of your monitor.) Stereochemistry

For a compound with multiple asymmetric carbons, an internal plane of symmetry indicates that the compound is achiral. However, it is still possible for a compound with multiple asymmetric carbons and no internal plane of symmetry to be meso. If there is only one asymmetric carbon, then the compound must be chiral.