Priorities belong to a channel and they determine what sets of ads have precedence over others. For instance, you can set rules so that House Ads (internal promotions) only appear if there's no advertiser to fill the slot.
Priorities work like a waterfall: the Kevel engine will attempt to fill any impressions that aren't captured by the highest priority with the next highest priority, and so on.
Priorities are also where you can define which ad selection algorithm is used for that respective priority.
Learn more here.