This is the 3151st most frequent Russian word.
мальчиков
"Мальчиков" means "boys" (genitive plural) in English.
In this sentence, 'мальчиков' is used in the genitive plural case to denote 'boys' as the object of the verb 'видел', meaning 'saw'.
Я видел группу мальчиков на площадке.
I saw a group of boys on the playground.
Here, 'мальчиков' is in the genitive plural case to express possession or association, indicating that the balls are intended for the boys.
У нас есть мячи для мальчиков.
We have balls for the boys.
In this context, 'мальчиков' would change to 'мальчиках', which is the prepositional plural case used with the preposition 'о', meaning 'about'.