In one isomer of tartaric acid, one asymmetric carbon atom is dextro rotatory and other is laevo rotatory- both rotating to the same extent in opposite directions. The net result is that this isomer becomes optically inactive and is called the "Meso" isomer.
Though the meso isomer has two asymmetric carbon atoms, the configuration of one carbon is the mirror image of the other, the net result being the molecule as a whole becomes asymmetric. This molecule is said to have a symmetric plane, which divides the molecule into two equal halves. The molecule becomes achiral. The optical inactivity of the meso isomer is due to internal compensation. This form is a single substance and not a mixture.