This is the 63rd most frequent Russian word.
детей
"Children" (genitive or accusative plural of ребёнок).
The word 'детей' is in the genitive plural case, as it is used in a construction denoting possession or quantity.
У меня трое детей.
I have three children.
Here, 'детей' is in the accusative plural case, as it serves as the direct object of the verb 'вижу' (see).
Я вижу детей в парке.
I see children in the park.
In this sentence, 'детей' is in the genitive plural case, used to indicate the intended recipients ('for the children') in prepositional usage.