The exposition of a story introduces the characters, the setting and the dramatic situation. It provides all the important information to start the story and ground the reader in the narrative's fictional universe. "Beowulf" is an epic narrative poem that tells the tale of how the hero, Beowulf, defeats the monster, Grendel. In the exposition, the poem introduces Grendel's history and the situation that brings the two together.
The story is set in Scandinavia, and it opens by introducing King Hrothgar and his lineage. Hrothgar and his descendants have been plagued by the monster Grendel invading the mead-hall each night and killing the soldiers, who try in vain to fight him. This information provides important context and characterization. It shows Grendel's apparent invincibility and shows what's at stake for the Danes if the demon is not defeated. The exposition also introduces Beowulf, who hears about the problem with Grendel and sees a challenge that he wants to pursue. He decides to go to Hrothgar to fight Grendel and prove his valor. This decision sets up the central dramatic situation, which is the fight between Beowulf and Grendel.