Caesar's ghost appears to Brutus in act 4, scene 3. In that scene, the ghost tells Brutus that he will see him at Philippi. This is significant because the Battle of Philippi is where Brutus and Cassius battle Mark Antony and Octavius to the death.
The Battle of Philippi is also a historic battle. You can read all about it from the link below. In Shakespeare's plays, the appearance of a ghost often foreshadows death. In this case, Caesar's ghost tells Brutus that he will see him at Philippi. This is a bad sign for Brutus, and the frightening vision is further reinforced by Cassius spotting ravens, kites, and crows in Philippi in act 5, scene 1. The appearance of birds of prey on a battlefield is a bad omen in Shakespearean plays, symbolizing imminent death and destruction.
The appearance of Caesar's ghost also rebukes Brutus's later stalwart pronouncement that he would never commit suicide in act 5, scene 1. For all intents and purposes, Brutus never intended to lose the war. However, the appearance of the ghost prior to the Battle of Philippi tells us that Brutus's cautious optimism has not served him well. In act 5, scene 5, Brutus finally reveals that Caesar's ghost appeared to him twice, once in Sardis and once in Philippi. At this point, Brutus is convinced that his "hour [has] come," and he enlists Strato's help in committing suicide.