The idea that an electronic transition occurs much faster than nuclear motion. For example, if you hit a molecule with a photon, driving it into the excited state, the electrons will rearrange their distribution in the orbitals without the nuclei having a chance to rearrange themselves. The resulting system is often not in equilibrium, and depending on the lifetime of the excited state, the nuclei may then rearrange (slightly) to accomodate the new electron distribution.

The description above seems more applicable to the Born-Oppenheimer approximation than to the Franck-Condon principle, although it is true that the Born-Oppenheimer approximation must apply for Franck-Condon factors to be calculated.

Franck-Condon factors give a quantitative value for the overlap between two vibrational states of an atom or molecule.