What is inert pair effect?

As elements get heavier, having spare electrons in a valence shell makes them less reactive than the same situation with the lighter elements. It's very noticeable for elements like lead where the oxidation state with two spare electrons is pretty stable. Not what you'd expect (you'd think the electrons would be trying to pair bond).

In this sense, the effect is known as the inert pair effect. It should be reactive oxidation state and yet it just isn't.