In two versions of the story, Clytemnestra has a pretty disturbing and totally symbolic dream. In Aeschylus' Libation Bearers, she dreams that she gives birth to a snake. (Stop for a second and think about how horrifying that would be.) After she gives birth to the snake, she breastfeeds it like a good mother should, only to discover that the snake is sucking blood along with breast milk. Clytemnestra is convinced that the snake represents Orestes, who's coming back to kill her.
In the Sophocles' version of the story, Electra, Clytemnestra sees Agamemnon in her dream. Her dearly departed husband takes his staff of leadership and stabs it into the ground. The staff then grows into a huge tree that overshadows all of Mycenae. Again, Clytemnestra interprets this to symbolize trouble heading her way in the form of her only son. She sees the sapling that grows from Agamemnon's staff to be Orestes, who will soon come back for vengeance and rule all of Mycenae, as his father did before him.