This is a Medinan surah which deals with the apocalyptic Day of Judgment (also called The Day of Resurrection) when the dead will be raised, judged, and sent to their eternal fates (heaven or hell). On the Day of Resurrection, there will be a massive earthquake and people will be thrown from their graves. The final judgment is described in this way: "On that day, people will come forward in separate groups to be shown their deeds: whoever has done an atom's-weight of good will see it, but whoever has done an atom's weight of evil will see that." In other words, even the smallest kindnesses and sins will be taken into account on the Day of Judgment. The impact of this on believers is to inspire them to do good, I think.
