Note 2880 — BOOK IV. AN ACCOUNT OF COUNTRIES, NATIONS, SEAS, TOWNS, HAVENS, MOUNTAINS, RIVERS, DISTANCES, AND PEOPLES WHO NOW EXIST OR FORMERLY EXISTED.
John Bostock translation
It is pretty clear that he refers to the numerous islands scattered over the face of the Baltic Sea, such as Dago, Oesel, Gothland, and Aland.