This is the 10107th most frequent Russian word.
басню
A fable.
The word "басню" (basnyu) is used in the accusative case of the noun "басня" to indicate the object of the action "рассказал" (told), meaning he shared a fable.
Он рассказал забавную басню на уроке.
He told a funny fable in class.
The word "басню" here again is in the accusative case, used as the direct object of "прочитал" (read), denoting the specific piece of literature.
Я прочитал интересную басню в книге.
I read an interesting fable in a book.
In this sentence, "басни" is plural and in the accusative case, showing that the children composed several fables.