Mail default value - In the absence of a custom field item, whatever you give as a mail default value will occupy that place. For example if your custom field is ‘Company name’ and you enter ‘Tenant’ as a mail default value, the campaign will use the word ‘Tenant’ if there is no item under Company name in the mailing list. 

Social default value- This is the same as mail default value except that it applies for social campaigns.