This is the 2390th most frequent Russian word.
осенью
"In autumn."
Here, 'осенью' is used as an adverbial modifier of time to indicate when the action ('starts falling') occurs.
Осенью начинают опадать листья с деревьев.
In autumn, leaves start falling from the trees.
In this sentence, 'осенью' specifies the time frame ('in autumn') for the action ('go for a picnic').
Мы обычно ходим на пикник осенью.
We usually go for a picnic in autumn.
The word 'осенью' describes the seasonal context ('in autumn') during which the subject ('scenes of nature') is observed.