Scandinavia, or the Scandinavian countries, are Denmark, Norway and Sweden. I sometimes hear people from the UK and the USA also include Finland and Iceland when referring to Scandinavia, but here in Scandinavia, these two countries (that are indeed very good friends of all three of the Scandinavian countries) are instead regarded as part of the larger geographical and cultural region referred to as the Nordic countries.

Most notably just means "especially" I think

The "notable" means they are "noted" or "well known" for something. "The Scandinavian countries are notable because nearly all their citizens speak English in addition to their native language."

The Scandinavian countries are Norway,Sweden,Finland and Denmark.