Well, you probably know that a single atom consists of a nucleus and a bunch of electrons in orbit around that nucleus. In this sense, you can say the electrons are located around the nucleus.
Now, when you bring a bunch of atoms close together, something interesting happens: Because the electrons start to feel not only "their" atom but also that of other atoms, the electron orbitals are modified:
Some orbitals, typically those with low energies, retain their atomic-like character, maybe with a few deformations, so electrons living in those orbitals still stay close to the nucleus. We call these "localized" electrons. Some orbitals, on the other hand, are smeared out over the entire material. These are extended or delocalized orbitals.
What he means by "inertia" is that ions are at least a factor of ~2000 heavier than electrons, and therefore they are very slow to react to changes in the electron states. Therefore, they don't follow every little fluctuation of the electrons. Their slowness averages out the tiny fluctuations.
This post imported from StackExchange Physics at 2014-04-01 13:30 (UCT), posted by SE-user Lagerbaer