This is the 392nd most frequent Russian word.
ними
The Russian word "ними" means "with them."
The word "ними" here is the instrumental case of "они" (they) used as the object of the preposition "с" (with), indicating accompaniment.
Я часто гуляю с ними.
I often walk with them.
The word "ними" is again in instrumental case to denote the object of the preposition "с", indicating association.
Мы разговаривали о них и часто с ними встречались.
We talked about them and often met with them.
"Ними" replaces "их" (them) in this instrumental case context to express engagement or interaction.