The email default value is the filler value that will appear in email campaigns if a field is empty. The social default value is the filler value that will appear in social campaigns or campaigns that are shared socially if a mapped field is empty. It is similar to the email default value, but for social campaigns.
For example, if the field $[UD:CITY]$ has no value and the social default value is City, then social campaigns will have City wherever the city merge tag is used.