This is the 13322nd most frequent Russian word.
мультфильмов
"Мультфильмов" means "cartoons" or "animated films" (genitive plural form).
The word 'мультфильмов' here showcases the genitive plural form referring to cartoons, used to describe the object of an activity happening (watching).
Я люблю смотреть мультфильмов по вечерам.
I love watching cartoons in the evenings.
In this context, 'мультфильмов' again is in the genitive plural case, utilized here as part of the possessive construct 'source of cartoons' meaning they provide something.
Для детей, мультфильмов является источником веселья.
For children, cartoons are a source of fun.
In this phrase, 'мультфильмов' is also in the genitive plural case, showing possession to describe an attribute about the 'world of cartoons'.