Grammatical case. I can only really describe it in German. If you take the sentence “The boy gives the man the apple”, it’s “Der Junge gibt dem Mann den Apfel”. “Der” is masculine form of “the” in the Nominative case. “Den” is the masculine form of “the” in the Accusative case. “Dem” is the masculine form of “the” in the Dative case. It’s subject, indirect object, direct object, respectively, if you know verbs. There’s also the Genitive case, which I didn’t go into here.

The reason it’s not sufficient to talk about subject, direct object, and indirect object though is because the grammatical case also goes beyond just a noun’s relationship to a verb, it’s also affected by prepositions. If you take the German sentence “I’m driving with the Man, but without the Apple” (I know, sort of a silly sentence), "ich fahre mit dem Mann, aber ohne den Apfel. The prepositions here, “mit” and “ohne”, dictate that the two masculine nouns in the sentence get the masculine form of “the” in the Dative case and Accusative case, respectively. The reason why some prepositions dictate certain cases isn’t clear to me. I just have the tables memorized :D