What is inert pair effect?

In the elements of 4th, 5th and 6th period of the p-block the electrons present in the intervening d and f-orbitals do not shield the r-electrons of the valence shell effectively.
As a result, ${{ns}^{2}}$ -electrons remain more tightly held by the nucleus and hence, do not participate in bonding. This is called inert pair effect.