What is an erg valve?

Pretty sure you mean "E.G.R. Valve" and not ERG. E.G.R stands for Exhaust Gas Recirculation Valve.

Instead of letting all of the gas out of the exhaust of the vehicle after the mixture is burnt in the compression cylinder, some of it is recirculated into the intake manifold.

This is done because not all of the vaporized gas is burnt out of the mixture that goes into the compression cylinder.

So this recycles some of the unburnt mixture, and reduces the amount of pollution emitted by your vehicle.