Molecules that have a plane of symmetry or a center of inversion are superimposable with their mirror image. They are said to be achiral molecules.

A plane of symmetry divides the molecule into two halves, which are mirror images of each other. 

The inversion center is a point with respect to which every element of the molecule has a symmetry.

The presence of elements of symmetry (planes, inversion centers, improper axes) make the molecules achiral.