Meso compounds are achiral compounds overall that still have chiral centers. These compounds are superimposed on their mirror images and are optically inactive even though they do contain chiral centers.
Meso compounds generally contain two or more identical chiral centers. They also have an internal symmetry plane that divides the entire compound into two parts. These two halves of the compound reflect each other and cancel out the chiral centers in the compound. As a result, the compound is optically inactive. To be optically active a compound must have a chiral center that does not cancel out. Cyclic and aliphatic compounds can both be meso compounds.