This achievement requires the player to travel to all Scandinavian cities added by the Scandinavia DLC. You need to discover 26 cities.
This can be done in your own truck or with a truck from a quick job, as long as you drive within city limits so you get the pop-up about discovering the city.
The cities needed for this are:
Denmark: Aalborg, Esbjerg, Frederikshavn, Gedser, Hirtshals, Copenhagen, Odense
Norway: Bergen, Kristiansand, Stavanger, Oslo
Sweden: Gothenburg, Helsingborg, Jönköping, Kalmar, Karlskrona, Linköping, Malmö, Nynäshamn, Örebro, Södertälje, Stockholm, Trelleborg, Uppsala, Västerås, Växjö
EDIT
In the 1.33 update was Kapellskär, Sweden added. This one might also be required for the achievement. As I've gotten it beforehand, I cannot test it. Any confirmation is welcome!