Marcellus and Bernardo are night watchmen (sentries) who have seen a ghost, the ghost of the man they think is the former King Hamlet in William Shakespeare's Hamlet. Before bothering his son, also named Hamlet, with this information, they call on Horatio to come and see what he thinks about the ghost and whether they should, indeed, tell Hamlet about it. Horatio is a reasonable man and a friend of Hamlet, so he is the logical choice.
In Act I scene i, it is night and they have all just seen the ghost. Marcellus asks Horatio to explain some things he has noticed and which have been bothering him. He rightly assumes Horatio knows more about these things than he does.
Good now, sit down, and tell me, he that knows,
Why this same strict and most observant watch
So nightly toils the subject of the land,
And why such daily cast of brazen cannon,
And foreign mart for implements of war;
Why such impress of shipwrights, whose sore task
Does not divide the Sunday from the week;
What might be toward, that this sweaty haste
Doth make the night joint-labourer with the day:
Who is't that can inform me?
The first question Marcellus asks Horatio is why there is suddenly such a "strict and most observant watch" being kept in Denmark now. Then he asks why the country is making so many cannons and importing so many other weapons. His third question is why the shipbuilders are working around the clock now (this question may be considered part of the second question), Finally, Marcellus wants to know what is happening--or about to happen--that has caused this flurry of war-like activity.
Horatio tells Marcellus what he knows--or has surmised from the "whispers" in the country. The former king, Hamlet, was an enemy to King Fortinbras of Norway. In their last battle, King Hamlet was challenged to do battle with Fortinbras, and Hamlet killed his foe. According to "a seal'd compact, / Well ratified by law and heraldry (a perfectly legal document), Hamlet got all of Fortinbras' territories when Hamlet killed Fortinbras. (If Hamlet would have lost the battle, Fortinbras would have gotten all of Hamlet's territories.)
Now Fortinbras' rather bold and reckless son (of the same name) has gathered a force of mercenaries (paid soldiers) and is hovering nearby, waiting to take back what his father lost. Thus, the military preparations.