Well, if you are talking about the artifact: "An incredibly rare, almost mythical artifact that functions as a psi-radiation amplifier." this is the description of the artifact given by the devs, I think they "left room" for maybe future DLCs or Stalker 3, maybe. If you are talking about the game in general I think it gets more clear at the end, something like you have to decide the fate of the Zone and for that you have to get to the "C-Consciousness" or so called "Hearth", maybe, that's how I understand that.
literally what is the "heart of chernobyl"
Well, if you are talking about the artifact: "An incredibly rare, almost mythical artifact that functions as a psi-radiation amplifier." this is the description of the artifact given by the devs, I think they "left room" for maybe future DLCs or Stalker 3, maybe. If you are talking about the game in general I think it gets more clear at the end, something like you have to decide the fate of the Zone and for that you have to get to the "C-Consciousness" or so called "Hearth", maybe, that's how I understand that.