This is the 3007th most frequent Polish word.
opolskim
"Opolskim" means "related to Opole" (a region or city in Poland) in the locative or instrumental case in Polish.
Here, "opolskim" is an adjective describing the region related to 'Opole', showing its locative case to indicate a location.
Mieszkam w województwie opolskim.
I live in the Opole Voivodeship.
In this sentence, "opolskim" modifies "lesie" to describe a forest situated in or associated with the Opole region, in the locative case due to the preposition 'w'.
W opolskim lesie jest wiele grzybów.
There are many mushrooms in the Opole forest.
Here, "opolskim" is used to attribute the festival to the Opole region, functioning as an adjective in the locative case following the preposition 'o'.