Can anyone explain what "Inert pair effect" is?

In fact, inert pair effect is a sort of phenomenon that "outermost electrons unwilling to be free out of the vicinity of atoms".
Taking C and Pb as examples:
In a C atom, the screening effect of the 1s electrons is rather good that the effective nuclear charge on the 2s electrons are small. Therefore those electrons can easily be excited to 2p orbitals for forming hybrid orbitals in hybridization and bond formation.
However, in a Pb atom, the screening effect of the 5d electrons is so poor that the effective nuclear charge on those 6s electrons is appreciably high. Therefore the 6s electrons are more difficult to be excited to the 6p orbitals.
Taking the oxide of lead as an example: PbO2 is very unstable that it readily decomposes back to PbO.
Though the 2s electrons are much closer to the nucleus in C than the 6s electrons do in Pb, the fact that electrostatic attraction force is a sort of strong field force makes the effect of nuclear charge override the effect of distance between electrons and nucleus.
Additionally, inert pair effect can also explain why mercury is so abnormal that it is a liquid under room conditions:
Electronic configuration of Hg: [Xe]4f14 5d10 6s2
So to speak, in the "sea" of electrons in mercury, there are much less free electrons than expected since, as stated before, those 6s electrons can hardly be free out of the mercury atoms. Therefore the metallic bond in mercury is much more weaker than other metals.