The normal human reaction is to evade the priority decision by doing a little bit of everything.
At the most basic level, the Product Owner is the person who decides what gets in the next. The term originates in Scrum, but it is not unusual to find the term being used in teams following other approaches, for example, Kanban, or their own hybrid processes.
In the early descriptions of Scrum, there was a tangible feel that the Product Owner really had the authority to make decisions. They were the owner. I still hope that is true, but more often than not the person playing ...