This is the 4432nd most frequent Polish word.
jesień
Autumn; fall.
In this sentence, 'jesień' is a noun referring to the autumn season.
Jesień jest moją ulubioną porą roku.
Autumn is my favorite season.
Here, 'jesieni' is the locative case of the noun 'jesień,' used to specify time or location.
W jesieni liście drzew zmieniają kolor na złoty.
In autumn, tree leaves change their color to gold.
The adjective 'jesienne' is derived from the noun 'jesień' and describes something related to autumn.