The symbolism of the dagger in "Macbeth" is that it represents Macbeth's bloody destiny, and Macbeth's vision of this dagger is one of the many hallucinations and visions that creates a motif of deception throughout the play. As he is about to murder Duncan, Macbeth notices a floating dagger, all bloodied and pointed in the direction of the chamber of the king.
This is not the last hallucination that Macbeth sees in the play. Later on, he catches a glimpse of the ghost of Banquo reclining in a chair at a meal, serving as a quiet reminder that Macbeth had slain his former friend. Lady Macbeth also succumbs to visions, as she starts sleepwalking and has a belief that blood has stained her hands so deeply that it can never be washed away, no matter how much water she uses.
Taking all of these hallucinations together, it is clear that the Macbeths feel a heavy sense of guilt, as all of these visions point them toward their own culpability in the death of others. This is one of Shakespeare's most violent plays, and even though most of the slayings take place offstage, one difference is that the characters come back out and describe the gore that has accompanied these deaths.