This is the 10978th most frequent Catalan word.
serbis
"Serbis" in Catalan means "Serbs" (people from Serbia).
In this sentence, 'serbis' refers to the people from Serbia, the ethnic group, used as a noun.
Els serbis tenen una rica cultura.
Serbians have a rich culture.
Here, 'serbi' (singular form related to 'serbis') is used as an adjective to describe the origin of the manual.
Aquest manual és serbi, ho poden veure en l'idioma.
This manual is Serbian, you can see it from the language.
In this case, 'serbis' again refers to the people from Serbia, specifically focusing on their historical context in a scholastic setting.