Lady Macbeth has just heard an owl shriek. According to medieval folklore, this foretold an imminent death. She compares the owl to a bellman, the town crier who went about town ringing a bell and making announcements. She describes the bellman as “fatal” because the crier often announced deaths. The bellman would bid good-night to anyone he passed in the street. The owl’s shriek is a stern, or cruel, good-night to Duncan since it’s forecasting his death.