Complex sentence: Molecular orbital (MO) theory yields a set of orbitals that have the symmetry of the molecule and that are delocalized over atoms, sometimes several; however, these orbitals can be transformed into an equivalent set of localized molecular orbitals.
Split into simpler sentences: Molecular orbital (MO) theory yields a set of orbitals that have the symmetry of the molecule; some of these MO's may be delocalized over several atoms. However these orbitals can be transformed into an equivalent set of localized molecular orbitals.