Campaign Influence in Salesforce is used to associate multiple influential campaigns to a single opportunity.
View influential campaigns from the Campaign Influence related list on the opportunity detail page. The Primary Campaign Source field on an opportunity detail page allows you to designate the most influential campaign for that opportunity.
1. Go to Customize –> Campaigns –> Campaign Influence.
2. Save the Campaign Influence.
3. You can add campaigns to Opportunities using Campaign Influence related list.