Calphurnia has shared a nightmarish dream she had with her husband, and as it turns out, he should have heeded her premonitions. In act 2, scene 2, he explains this dream to Decius when he stops by:
Calphurnia here, my wife, stays me at home.
She dreamt tonight she saw my statue,
Which, like a fountain with an hundred spouts,
Did run pure blood. And many lusty Romans
Came smiling and did bathe their hands in it. (II.ii.80-84)
Ironically, Decius (who is in on the murderous plot against Caesar) convinces Caesar that there is nothing to worry about and that the dream really means that people want to symbolically bathe in Caesar's blood as a sign of his approval. Caesar really likes this interpretation, so he follows Decius to his death because of his vanity.
Calphurnia's dream does, unfortunately, come true and in the way she'd feared. Caesar is surrounded by the conspirators and stabbed to death, his body running blood just as in Calphurnia's dream. After the murder is complete, the group immediately begins discussing what a gift they have given to Caesar by killing him. After all, they reason, now he doesn't have to worry about dying for another twenty years, and they have given him a great "gift" by sparing him the torture of living. They imagine that their actions will become so epic that many plays will be created to act out the valiant efforts they have taken to rid the world of Caesar. These reflections are symbolized in Calphurnia's dream as well; their lust for power leaves the men "smiling" as they are covered in Caesar's blood.