polarization density
An insulator has no free charges to conduct, but its molecules are not indifferent to a field: their positive and negative charges shift slightly apart, or their permanent dipoles swing into line. Polarization density is the net electric dipole moment per unit volume that this microscopic separation produces.
The polarization P is the electric dipole moment per unit volume of a material. Its macroscopic effect appears as bound charge: a nonuniform polarization leaves a bound volume charge density rho_b = -div P, and the surface where P ends produces a bound surface charge sigma_b = P·n. These bound charges are genuine sources of electric field, exactly like free charges, but they are locked to the molecules rather than free to move. Polarization enters the displacement field through D = epsilon_0 E + P.
Polarization is the electric twin of magnetization M. In a linear dielectric it is proportional to the field, P = epsilon_0 chi_e E, with chi_e the electric susceptibility, related to the dielectric constant by epsilon_r = 1 + chi_e. Beyond the linear regime lie the rich phenomena of ferroelectrics (which retain polarization with no applied field) and nonlinear optics (where P depends on E^2 and higher powers, generating new frequencies of light). The bound charge -div P is why a uniformly polarized slab has charge only on its faces, not in its interior.
A slab uniformly polarized along its thickness has div P = 0 inside, so no bound charge in its bulk; but where P meets the two faces it deposits surface charge sigma_b = +P on one face and -P on the other. The slab thus acts like a pair of charged sheets — a built-in field with no free charge anywhere.
P is dipole moment per volume; it yields bound charge rho_b = -div P and enters D = epsilon_0 E + P.
Bound charge is as real a source of E as free charge — the label 'bound' means it cannot flow away, not that it is fictitious. Only a NON-uniform P gives volume bound charge; a uniformly polarized body carries bound charge purely on its surfaces.