This is the 13339th most frequent Polish word.
ostrołęckim
"Ostrołęckim" means "related to Ostrołęka," a city or region in Poland.
Here, 'ostrołęckim' is used as an adjective, describing the location of the museum as being in Ostrołęka, following the instrumental case.
Spotkaliśmy się w ostrołęckim muzeum.
We met in the Ostrołęka museum.
In this sentence, 'ostrołęckim' attributes the market square to Ostrołęka while following the locative case in Polish.
Czytałem ciekawy artykuł o ostrołęckim rynku.
I read an interesting article about the Ostrołęka market square.
The word 'ostrołęckim' determines the tournament as related to Ostrołęka, applying the locative case.