The Orestaia is a group of three separate plays written by author Aeschylus. Agamemnon begins right at the tail end of The Iliad, which is the story of the Trojan War. Agamemnon has been away from home for a decade fighting to get his brother's wife back, and the watchman opens the play sighing wistfully for his master's return. I've always interpreted these opening lines somewhat differently than the previous educator—as we know by the end, Agamemnon's wife Clytemnestra is not a savory character. She has been scheming with Agamemnon's cousin to kill him in punishment for his father Atreus's sins (there is a long and complicated backstory to all of that, but in general the theme is the sins of the father are paid for by the son).
The watchman laments the last ten years, saying he has taken his orders from a "woman with a man's heart." This alludes to Clytemnestra's cruelty and habit of treating those below her like dogs. It also gives the reader a hint into Greek society—Clytemnestra has gone against social convention by choosing her husband (she has married her co- conspirator) and ruling while Agamemnon is away. The watchman, in only a few sentences, gives a pretty clear picture of the state of the kingdom: a wicked and cruel mistress, a missing king, and general anxiety about the state. Agamemnon's return gives the watchman hope, as he is a seen as a good man and good king.