Basically, the online version is trying to be more faithful (or so they hint) to the Arcady & Boris Strugasky "Roadside Picnic" novel (also titled "Stalker", hence the name), where the S.T.A.L.K.E.R. series was a very modern take and little to do with it (besides the "something wrong happens and there's this huge uninhabitable area").
If you're unfamiliar with this SF classic, basically one day, aliens (or so we supposed, no one saw anything) drop down somewhere, everyone panics, then they leave, and all that remains is Zones where... artefacts remain, and things happen (things like having small patches of ground where there's like 1000G gravity, smashing you into thin paste, or blue spider webs that simply drops you dead after one hour if you touch it). Specialists, called Stalkers, go there to recover stuff for the science teams that don't understand any of it, and furnish a lot of black market stuff.
That's what they're referring to when they talk about the Zone. Basically you go into a Zone filled with weird stuff, and try to get some loot.