To import a default value to a field:
On the Import Mapping page, check the Enable Default Value Import. This will present a column to data enter or choose from a menu the value that will be imported into that field.
If you put a default value for a field that you have also mapped to a field from your import file, the default value will only be imported for a contact if there is no data in the corresponding field in your import file.
This can be a quick way of adding data that applies to all contacts in a file, such as lead source or team.