What is Stoichiometric numbers, extent?

A reaction is written $0 = \sum_i \nu_i\,\mathrm{A}_i$, where the stoichiometric number $\nu_i$ of species $\mathrm{A}_i$ is negative for a reactant and positive for a product: in $\ce{N2 + 3H2 -> 2NH3}$, $\nu(\ce{N2}) = -1$, $\nu(\ce{H2}) = -3$, $\nu(\ce{NH3}) = +2$. In a closed system where this reaction alone takes place, the amounts change as

which defines the extent of reaction $\xi$ (in moles), zero at the start and the same for every species.