Question

what is called inert pair effect

Answer 1

The inert pair effect is the tendency of the two electrons in the outermost atomic s-orbital to remain unionized or unshared in compounds of post transition metals.
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Answer 2

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$\color{Red}{Inert\: Pair\: Effect}$ : The tendency of ${ns}^{2}$
electron pair to participate in bond formation decreases with the increase in atomic size. Within a group, the higher oxidation state becomes less stable with respect to the lower oxidation state as the atomic number increases. This trend is called 'inert pair effect'. In other words, the energy required to unpair the electrons is more than energy released in formation of two additional bonds.