You can find the answer to your question right here on eNotes at the link below. You will see that this play is an absurdist approach to Shakespeare's Hamlet. In Hamlet, Rosencrantz and Guildenstern are very minor characters, friends from Hamlet's youth. Claudius calls them to Denmark to spy on Hamlet under the guise of finding out why he is depressed. Hamlet figures this out and in a counterplot, they are sent back to England and killed.

In the Stoppard play, Hamlet is a minor character and Rosencrantz and Guildenstern and the major characters. They are still friends from Hamlet's youth, but in the play, they are given personalities. The way they interact with Hamlet in Stoppard's play and the way they interact with each other and with their task illustrates the existential themes of the play, the absurdity of the human condition, death, life, art, etc.