A Salesforce Campaign is a set of Campaign Members(Leads and Contacts).

How to add multiple Leads and Contacts to a Campaign in Lightning Experience in Salesforce? – https://www.infallibletechie.com/2018/04/how-to-add-multiple-leads-and-contacts.html

Campaign Influence in Salesforce – https://www.infallibletechie.com/2015/11/campaign-influence-in-salesforce.html

Salesforce Help Article – https://help.salesforce.com/articleView?id=sf.campaigns_def.htm&type=5 

Trailhead – https://trailhead.salesforce.com/en/content/learn/modules/campaign_basics