Complex sentence: In physics and mathematics, mirror symmetry is a surprising relation between two (usually) six-dimensional Calabi-Yau manifolds whose shapes look very different geometrically, but nevertheless they are equivalent if they are employed as hidden dimensions of string theory.
Split into simpler sentences: In physics and mathematics, mirror symmetry is a surprising relation that can exist between two Calabi-Yau manifolds. It happens, usually for two such six-dimensional manifolds, that the shapes may look very different geometrically, but nevertheless they are equivalent if they are employed as hidden dimensions of string theory.