Who's there is the opening question in Hamlet. It is a significant question. Bernardo asks the question. Francisco responds with the same question. This is a relevant question because the guardsmen have been guarding the castle of the King of Scotland. For many nights now, these guardsmen have been seeing King Hamlet's ghost. Now they are tense and on edge, assuming anyone who comes up in the dark is King Hamlets's ghost.
Marcellus and Bernardo are discussing the ghost. Marcellus asks Bernardo has he seen the ghost yet? Marcellus adds that Horatio thinks it is their imagination. Then the ghost appears and Horatio is just as afraid as the other guardsmen.
The guardsmen bring young Hamlet to see the ghost. When young Hamlet sees the ghost, he realizes it is his father. The ghost tells Hamlet how he was murdered at the hands of Claudius and commissions his son to avenge his death.
The play is centered around this significant appearance of King Hamlet's ghost. Now, young Hamlet is driven by a spirit of revenge, thus the play progresses around young Hamlet and his desire to avenge his father's death.