I assume you are talking in this question about what happens in Act I, Scene 2. If so, the answer is that Claudius tries to keep Denmark from having to have a war with Norway. He does this by sending a letter with Cornelius and Voltemand to the King of Norway. In the letter, he asks the king to restrain his nephew, Fortinbras.
Fortinbras has been getting troops together, seemingly to attack Denmark. He wants revenge for the fact that Hamlet (the late king) has killed his (Fortinbras's) father and taken some Norwegian land in a war.