This is the 987th most frequent Polish word.
wiosną
"In spring."
In this sentence, 'wiosną' functions as a temporal adverbial expression indicating when the described activity takes place.
Często spotykamy się na piknikach wiosną.
We often meet for picnics in the spring.
'Wiosną' here is used similarly to indicate the period during which the birds' return happens.
Ptaki wracają z dalekich krajów wiosną.
Birds return from distant countries in the spring.
Here, 'wiosną' is an adverbial form of the noun 'wiosna', indicating the time when the action occurs.