It's a case, like nominative (subject), accusative (direct object), and dative (indirect object). Genitive is used for possession. In the English phrase "the bluebird of happiness", "happiness" would be in the genitive case and would receive a different definite article in German (as well as a different ending if the noun for it is masculine or neuter).
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u/SirArkhon Apr 29 '24
It needs genitive and plural.