"It is the biggest island of the country."
Translation:Es ist die größte Insel des Landes.
No, "des" is the definite article in the second case, singular, neuter.
Nominativ: das Land
Genitiv: des Landes
Dativ: dem Land
Akkusativ: das Land
A tip: Most English-speakers don't learn German in the same way as native Germans. The usage of "first/second/etc." cases is not a common terminology amongst modern English-based German language resources, and is in contradiction to the order in which the cases are introduced in Duolingo and many other resources. I would recommend simply referring to them by their names :)