It is a pronoun that introduces a subordinate clause within a main clause, also known as a relative clause. They are used to add some type of information about the object or person referred to by the pronoun.
who: is used to refer to people, it can play the role of subject or object of the sentence.
that: it can be used to refer to people, animals or things.
which: Is used exclusively to refer to animals and things. You can play the role of subject or object indifferently.
whom: Is hardly used and is used more on a formal occasion, is used when the pronoun who is the object of the verb or is accompanied by a preposition.
where: Is used to refer to places, and can be omitted only in the case of being accompanied by a preposition.
Defining relative clauses give essential information to define or identify the person or thing we are talking about. The pronouns are the same as the relative pronouns.
provide some additional information that is not essential and may be omitted without affecting the contents of the sentence.
who, which and whose: We can use who to talk about people, which to talk about things and whose to refer to the person or thing that something belongs to.
We can use which with a preposition to talk about places and times. In these cases it's more common to use where or when instead of which and the preposition.