What is inert pair effect?

Inert pair effect is the name given to tendency of electrons present in the outer most s sub shell of post-transition metals, like Pb, Bi etc. to not undergo ionisation, and to remain unshared. This is most stated in Groups 13, 14 and 15.

A possible given explanation is that due to presence of f-subshell electrons are too diffused, due to the shape of the f orbitals, to effectively shield the s electrons from the pull of the nucleus.

A good example would be the elements of Group 13. Aluminium in +1 state is unknown, and the stability of +1 oxidation state increases as we go down the group. Thallium is most stable in +1 oxidation state, and compounds of Thallium in +3 oxidation state are known to be very strong oxidising agents, and have a tendency to reduce quickly to +1 state