This is the 6423rd most frequent Russian word.
шутку
A joke.
The word 'шутку' refers to a 'joke' in the accusative case, being the direct object of the verb 'сказал' (said).
Он сказал шутку.
He told a joke.
Here, 'шутку' is again in the accusative case, serving as the direct object of the infinitive verb 'понять' (to understand).
Не всегда легко понять шутку.
It's not always easy to understand a joke.
In this sentence, 'шутку' appears in the accusative case as the object of the verb 'слушать' (to listen to).