Here's what the arrangement entails:
* Hamlet asks the First Player to perform a specific play. The play is designed to be a near-identical reenactment of the murder of Hamlet's father, King Hamlet, by Claudius.
* The play's purpose is to test Claudius's reaction. Hamlet wants to see if Claudius will react with guilt or panic during the performance, which would provide strong evidence of his guilt.
* Hamlet instructs the First Player to deliver a specific line (the "mousetrap speech") that directly accuses the murderer of the King.
* Hamlet offers to pay the First Player and his troupe for their services.
The arrangement is crucial to the play's plot as it sets in motion the events that lead to Claudius's eventual downfall.