High Level PrioritisationWith larger Product Backlogs, it's generally good practice for the Product Owner to start prioritising at a high level.
This could be a "MoSCoW" (Must have, Should have, Could have and Won't have) approach or could be that the Product Owner just selects what stands out at them as the most important User Stories to begin with.
Either way, this ensures that the more important User Stories are addressed at first.
Prioritising is incredibly important part of the Agile process. Without prioritisation the team does not have direction, delivery is not clear and value of the product cannot be visualized.