Priority level is a flexible rating system that lets you assign priority to a person in your nation. It can be very useful for targeting outreach once you've filtered for an audience. Priority level can also be used to communicate the value of a person in your nation to other control panel users. Here is more info on how it can be assigned and changed - http://nationbuilder.com/whats_the_priority_level_feature_and_how_do_i_use_it