This is the 837th most frequent Czech word.
námi
"Us" (instrumental case).
In this sentence, "námi" is used as an instrumental case form of the pronoun "my" (we), meaning "with us".
Pojďte s námi do kina.
Come with us to the cinema.
Here, "námi" is used to indicate a spatial or figurative relationship, translated as "between us".
Mezi námi je velký rozdíl.
There is a big difference between us.
In this case, "námi" again marks a relationship, but it specifically highlights a group of two individuals, as "between the two of us."