The narrator of Beowulf is one of those godlike narrators who sees everything and can skip around between different characters and between the past, present, and future. In fact, you might get a little bit frustrated with this, because the narrator often spoils the story for you. More than once, the narrator steps aside to remind you that Beowulf may seem like an invincible hero, but eventually God is going to decree that he'll be defeated.
At other times, the narrator jumps back in time to explain why things are unfolding as they are. For example, when Beowulf chooses to fight Grendel in hand-to-hand combat, the narrator explains that this is lucky, because Grendel is impervious to edged weapons like swords. That's not something that anyone in the story actually knows – in fact, none of them even find out. But the narrator gives us this extra explanation about some of the supernatural elements in the story so that we know more than the characters do.
The narrator also uses flashbacks to explain the history between different men, such as Beowulf and Hrothgar, or different tribes, such as the feuding Geats and Swedes. In fact, this kind of narration makes it easier for us to understand Beowulf as we read it, because the narrator is always making connections between past history and the battles Beowulf is fighting at any given moment.