которогоФорма относительного местоимения, объект глагола
The colleague whom I met yesterday will join us.
Коллега, которого я встретил вчера, присоединится к нам.
Определения
The objective form of “who” used in direct or indirect questions to refer to the person or people receiving an action.
Whom did you invite to the ceremony?
The objective form of “who” introducing a relative clause referring to a person or people who are the object of a verb or preposition within that clause.
The colleague to whom I sent the report will respond tomorrow.
Introduces a nominal clause in indirect questions to indicate a choice between a limited set of options (nominal clause).