Like Agamemnon, Part 1 of Aeschylus's Oresteia trilogy, Libation Bearers takes place in Argos, a city in the Peloponnese, a large peninsula in the south of mainland Greece. ( Here's a map.) The difference is that Libation Bearers takes place some years after the events of Agamemnon. We aren't told how many years exactly, but enough time has passed for Orestes to come to manhood.
Even though Clytemnestra and Aegisthus are still ruling Argos, they are unpopular (this actually isn't that different from the end of Agamemnon), as we learn from the Chorus in the opening scene at the tomb of Agamemnon. The fact that the Chorus expresses these thoughts at the tomb of Agamemnon is significant. This tomb is a little ways out of town, so characters can speak much more freely there than they would be able to if they were in downtown Argos. Thus, standing by the tomb of their father both reconnects Orestes and Electra to their past and also gives them the freedom to plot their revenge.
After the opening scene, the action of the play shifts slightly to the main gate of the palace in Argos. This setting echoes the action of Agamemnon. At the end of Libation Bearers, Orestes opens the palace gates to reveal himself standing over the dead bodies of Clytemnestra and Aegisthus. A similar thing occurs at the climax of Agamemnon, when Clytemnestra opens the gates to show herself standing over the bodies of Agamemnon and Cassandra.
The shared setting between Libation Bearers and Agamemnon serves as a visual metaphor of the Oresteia trilogy's theme of the vicious cycle of revenge. (As you will see if you read it, the third play, Eumenides, changes the setting to Delphi and then Athens; this parallels its theme of attempting to break that bloody cycle.)