This is the 116th most frequent Latvian word.
valstu
"Valstu" is the genitive plural form of "valsts," meaning "of countries" or "of states."
In this sentence, 'valstu' is the genitive plural form of 'valsts' (country), indicating possession ('of the countries').
Valstu karogi plīvoja vējā.
The flags of the countries were flying in the wind.
Here, 'valstu' in the genitive plural form describes the grouping concept ('group of countries').
Katras valstu grupas kultūra ir unikāla.
The culture of each group of countries is unique.
'Valstu' in this context signifies that the borders are related to multiple countries (in a possessive sense).