This is the 834th most frequent Serbian word.
крагујевцу
"Крагујевцу" is the dative or locative case of "Крагујевац," meaning "to Kragujevac" or "in Kragujevac," a city in Serbia.
The preposition "у" indicates a location, and "Крагујевцу" is in the dative case to agree with it.
Јуче сам био у Крагујевцу.
Yesterday, I was in Kragujevac.
The preposition "о" is followed by the locative case, hence "Крагујевцу" is in the locative case.
Причам о Крагујевцу.
I am talking about Kragujevac.
The preposition "Крагујевцу" is in the dative case, which is used to indicate the indirect object of the sentence.