This is the 674th most frequent Russian word.
сына
"Сына" means "of the son" (genitive case of "сын").
"Сына" appears in the genitive case to indicate a possessive relationship after the word "своего".
Иван гордится успехами своего сына.
Ivan is proud of his son's achievements.
Here, "сына" is used in the accusative case to denote the direct object of the verb "встретил".
Я встретил сына друга в магазине.
I met a friend's son at the store.
In this sentence, "сына" is in the genitive case to indicate possession, as dictated by the structure of the phrase "есть сына".