This is the 8678th most frequent Polish word.
radzynia
"Radzynia" does not have a direct meaning in Polish; it might be a proper noun, such as a place name.
Here, 'Radzyniu' is used as the locative case of the proper noun 'Radzyń', a town name, after the preposition 'w' (in), indicating location.
Czy byłeś kiedyś w Radzyniu Podlaskim?
Have you ever been to Radzyń Podlaski?
Here, 'Radzynia' is used as the genitive case of 'Radzyń', indicating possession/reference to history closely related to the town.
Lubię historię Radzynia i jego zabytki.
I like the history of Radzyń and its monuments.
Here, 'Radzynia' is used as the ablative (or genitive dependent on context) case of 'Radzyń', signifying the origin in the context of movement.