Campaign priorities determine your bids according to the rule that if the highest priority campaign is out of the budget, the lower priority campaign bids.

Explanation: If the campaign with the highest priority runs out of the budget, the next lower priority campaign will place the bid. Continuing with the previous example, when the budget for the High priority campaign is used up, the bid from the Medium priority campaign will be used.

Read more here: https://support.google.com/adwords/answer/6275296