The GeoIP lookup function allows for automatic collection of country information on form submissions. Manual entry on the forms will always override automatic country detection. 

If the "country" field is present on a form it will be pre-filled based on the contact's IP address.

When GeoIP is enabled and there is no "country" field on the form form it will be automatically detected and attached to the contact in the background.

GeoIP removes the need to add a country field on your forms by automatically collecting country information to a high degree of accuracy for each form submission.

County level location is critical for CASL (Canadian Anti-Spam Legislation) compliance. All users entering your database via forms, are automatically assigned their correct country which ensures that Canadian contacts are marked as such and will be managed according to CASL and non-Canadian contacts won't be subject to CASL.

The option is available from the Account Settings page in the Form section:

GeoIP will not work for manually entered contacts, for imported contact lists or for contacts submitted via the API.

See list of ISO country codes and names

Related articles about working with forms.