The 'Priority' setting can be used to prioritize certain line items over others, for example setting paid line items higher than internal promotional messages (trailers, etc).
All Line Items are assigned Standard priority by default, but they can be changed to Urgent, High, Low, or Very Low. Outside of exclusions that disqualify a line item from being delivered, campaign priority is the first setting considered for selection.
A higher priority line item will always be selected over a lower priority campaign.
You can also find more information on how ads are selected here: How are Ads Selected for an Episode? (Ad Selection Guide)