The chorus chant that they have been sent to mourn at the grave of Agamemnon by Clytaemestra because she has been tormented by bad dreams and hopes in this way to appease her dead husband's spirit. Clytaemestra believes that libations at the tomb will protect her from retribution, but, the chorus say, nothing can wash away the blood-guilt of a murder; all such crimes inevitably are punished.
The first choral ode establishes the moral and emotional background of the play by renewing the oppressive, pessimistic mood in which Agamemnon concluded. Despite her confident assertion that no more blood would be shed, Clytaemestra is beginning to realize that she will have to pay for her crime. Orestes, it will be seen, is not eager to kill his mother, but Apollo has commanded it, and he will do his duty. It appears that the cycle of violence and murder will go on forever unless an acceptable moral solution is found.
The women of the chorus are captives, but their bitterness and desire for revenge against the murderers seem more intense than that of Orestes and Electra This is because they are symbolic spokespeople for the primitive and absolute moral law that is responsible for the moral dilemma in the trilogy — blood must be paid for with more blood, or, in the words of the Old Testament, "an eye for an eye."
An interesting technical point here is that the details of Clytaemestra's dream are withheld until later in the play, where they will have more dramatic effect.