As per CliffNotes, Horatio receives letters from a sailor sent by Hamlet. The first letter tells Horatio that pirates beset the ship on which Hamlet was being carried to England. In the ensuing battle, the pirates took Hamlet captive; they treated him well and brought him back to Denmark. He has, in return, promised to do them a favor. The other letters, says Hamlet's first letter, are for Horatio to deliver to the King. After he has made the delivery, Horatio is to come immediately to meet Hamlet; Hamlet tells his friend that he has much news to share.
See Hamlet's letter to Horatio at SparkNotes.
The Tragedy of Hamlet, Prince of Denmark, often shortened to Hamlet, is a tragedy written by William Shakespeare at an uncertain date between 1599 and 1602. See Shakespeare's Hamlet with explanatory notes and study guide, all at Shakespeare Online.
Check out the ten best Shakespeare plays of all time, sahred by Time Out London.