I certainly understand your questions on trying to make sense of the sourdough terminology. Long story short: levain is flour and water that has been inoculated with active sourdough starter and left to ferment until active for use. You might also hear this being stated as "feeding" a starter. It is the chicken and the egg - the starter is levain, and it makes more levain. Levain is a french term more attributed to bread making. I use the term levain here because the dough is basically bread, having been made with a large percentage of sourdough (20% of flour). It is not true Neapolitan pizza dough.
In the case of Neapolitan dough being made with sourdough starter (called crisceto in Italian), the normal proportion would be 3-5% of the amount of water. It would then be left for around 18 hours to ferment at room temp, most of that time being balled. You should take a look at Craig's thread on making dough at the top of the Neapolitan forum for more info on the complete process.