This is the 14775th most frequent Polish word.
radosny
Joyful.
The word 'radosny' is used here as an adjective to describe the boy's cheerful disposition.
To jest bardzo radosny chłopiec.
This is a very joyful boy.
Here, 'radosny' is an adjective used to characterize the event as joyful.
Wczorajsza impreza była radosnym wydarzeniem.
Yesterday's party was a joyful event.
In this sentence, 'radosna' (a form of 'radosny') describes the nature of the reaction, showing its positive and cheerful quality.