He basically gives him his resume, or all the heroic things he has done in the past, such as: he has never been defeated, he has saved many people, he talks about how he lost the swimming match to his friend Brecca because he stopped to help him against sea monsters which attacked him, etc.
These boasts are here for Beowulf to show to Lord Hrothgar that he is worthy of defending his kingdom against the monster Grendel.