This is the 3276th most frequent Russian word.
осень
Autumn; fall.
Here, 'осень' is used as a noun referring to the season of autumn.
Осень — моё любимое время года.
Autumn is my favorite season.
In this sentence, 'осень' is in its prepositional case 'осени'. It denotes the time and context related to autumn.
В осени есть что-то волшебное.
There is something magical in autumn.
The word 'осень' is used in an adverbial sense to specify the recurrence of the action during the autumn season.