This is the 3137th most frequent Russian word.
кусочек
A small piece or bit.
Here, 'кусочек' means 'a small piece' and refers to a part of chocolate that was eaten.
Я съел кусочек шоколада.
I ate a piece of chocolate.
In this sentence, 'кусочек' signifies a segment or portion of bread, emphasizing the diminutive and endearing aspect.
Можешь дать мне кусочек хлеба?
Can you give me a piece of bread?
Here, 'кусочки' (the plural of 'кусочек') indicates fragments or small parts, showing a result of an action or division.