This is the 8726th most frequent Polish word.
rezerwistów
"Rezerwistów" means "reservists" (individuals in a military reserve).
Here, 'rezerwistów' functions as the plural form of the genitive case used to indicate a meeting involving reservists.
W poniedziałek odbędzie się spotkanie rezerwistów.
On Monday, a reservists' meeting will take place.
In this sentence, 'rezerwistów' is used as the plural form in the genitive case to denote the group involved in assisting with the event.
Grupa rezerwistów pomagała przy organizacji wydarzenia.
A group of reservists helped with organizing the event.
Here, 'rezerwistów' in the plural genitive case represents those who were invited to lunch after exercises.