This is the 197th most frequent Russian word.
котором
"Котором" means "in which" or "on which" (prepositional case, masculine/neuter singular).
The word 'котором' is used as a relative pronoun in the prepositional case referring to the noun 'дом'.
Это дом, в котором я вырос.
This is the house in which I grew up.
Here, 'котором' is a relative pronoun in the prepositional case referring to the noun 'фильм'.
Мы обсуждали фильм, о котором все говорят.
We discussed the movie which everyone is talking about.
In this instance, 'котором' serves as a relative pronoun in the prepositional case referring to the noun 'день'.