What is Extent of reaction, progress table?

The extent of reaction $x$, in moles, counts how many times the reaction, as written in the balanced equation, has taken place: when the extent is $x$, a species of coefficient $\nu$ has been used up (reactant) or formed (product) in the amount $\nu x$. The progress table gives, under the equation, the amount of every species in the initial state, at an extent $x$, and in the final state.