The game is based on a classic Russian sci-fi novel, "Roadside Picnic" (by Arkadiy and Boris Strugatsky, rather recommended). To summarize the game's backstory:
In 2008, a bright flash is seen coming from the Chernobyl Alienation Zone. After this event, weird things start appearing in it - like mutated animals, radiation pockets, and anomalies, areas where laws of physics seem to function differently. The Zone is cordoned off by the military. Exploration of the Zone, both legal and illegal, finds that certain objects found near anomalies seem to have the same physics-altering qualities, like making bullets curve away from the owner, or boosting their health. The high demand, and resulting high prices for these artifacts caused rogue explorers to pour into the Zone. These people were later dubbed Stalkers, in honor of the main character of the Strugatskys' novel.