This is the 605th most frequent Polish word.
białymstoku
"Białymstoku" is the locative case of "Białystok," referring to the city of Białystok in Poland.
Here, 'Białymstoku' reflects the locative case, used after prepositions indicating location, such as 'w' (in).
Mieszkam w Białymstoku.
I live in Białystok.
The form 'Białegostoku' represents the genitive case, used here after the preposition 'z' (from) indicating the origin of the action.
Wracam z Białegostoku.
I am returning from Białystok.
In this sentence, 'Białymstoku' again is in the locative case because it follows the preposition 'w' indicating the place of action.