This is the 16429th most frequent Polish word.
wczorajszej
"Of yesterday" (feminine form).
The word 'wczorajszej' here is used as an adjective in the genitive case, describing 'rozmowy' (conversation) to indicate it happened yesterday.
Nie pamiętam szczegółów wczorajszej rozmowy.
I don't remember the details of yesterday's conversation.
The word 'wczorajszej' is an adjective in the genitive case, used to describe 'gry' (game) and to indicate that it took place yesterday.
Czy widziałeś wyniki wczorajszej gry?
Did you see the results of yesterday's game?
In this example, 'wczorajszej' acts as an adjective in the genitive case, modifying 'decyzji' (decision) to specify its occurrence as yesterday.