When your audiences are narrow in focus, e.g. new customers and returning customers, the chance of a customer being in more than one audience is highly unlikely.
However, with audiences that are broad in focus, it is possible that a customer could be a member of both audiences and therefore potentially see both experiences.
Say, for example, you created an experience for customers in London and another for customers that had visited your site before but had never placed an order.
A returning visitor from London, yet to place an order on your website would fall into both audiences. So now we need to decide what experience to show. This is where priority comes in. By dragging and dropping experiences you can order them into a priority list. Customers that fall into both audiences will be shown the experience at the top of the list.