Hidden profile fields are fields or data points that you can have for your panel member profile that the member can not see or update. The field is accessible to update / edit / sample on only for the admin. This is when you dont want the member to know the field is he is being included / excluded in the sample on.
To mark the profile field as hidden, go to:
Login » Communities » (Select Community) » Edit » Members » Profile » Global Community Member Profile
Click on Add New Profile Field, turn on the option for Private/Hidden Field
Yes. The hidden fields are included in the import, so you as a admin will have access to the data for those fields and you can also sample the members on those however the member will not see those fields in the portal and hence can not edit those fields.
The feature is very useful when you have some kind of private information of the panelist that you don't want them to update easily without any authentic means. For instance Social Security Number, Voter ID, License information etc.