This is the 203rd most frequent Czech word.
kterých
"Of which" or "of whom."
In this sentence, 'kterým(i)' (alternative form 'kterých') is a relative pronoun in the locative case, linking the antecedent 'people' with the descriptive clause.
Lidé, mezi kterými jsem vyrůstal, byli velmi přátelští.
The people among whom I grew up were very friendly.
Here, 'kterých' is used as a relative pronoun in the genitive case for 'houses', connecting the main clause with additional information.
To jsou domy, o kterých jsem ti vyprávěl.
Those are the houses I told you about.
'Kterých' functions as a relative pronoun in the genitive case, specifying the reasons referred to in the subordinate clause.