The invocation of the muse in the first twelve lines of the poem suggests that the poem will focus on both Odysseus's struggles as well as his men's perversity and disobedience and how this is what prevented them from coming home.  Despite how hard Odysseus fights to achieve their "safe return," their own bad behavior makes it impossible.  This seems as though the poem is, in part, going to focus on how Odysseus's obedience to the gods will make it possible for him to return to Ithaca at last, in addition to how his crew's choice to disobey ruined their chances, emphasizing the importance of obedience to the gods and their laws at all cost.  In general, those people who disobey the gods' laws are punished harshly in the poem, including the suitors who exploit and abuse religious ideas about hospitality.  Such emphasis gives the poem religious importance, as Odysseus's behavior can serve as a model for others, as well as being a form of entertainment for its audience.

The invocation is the narrator calling for help from the muse to tell his story of Odysseus and his travels back home to Ithaca after the Trojan War. 

"Many cities did he visit, and many were the nations with whose manners and customs he was acquainted; moreover he suffered much by sea while trying to save his own life and bring his men safely home"

From this we know that the tales will be told of where he went, whom he met, and the suffering involved.  Then in the last lines of the invocation he asks the muse to help share the story.

"Tell me, too, about all these things, O daughter of Jove, from whatsoever source you may know them."