Relative clauses are used to give additional information about something or someone without starting other sentence. A relative clause usually comes after the noun from the principal clause it is related to.
Relative pronouns (which, that, who, where, whose, whom) are used to introduce a defining relative clause.
We should help somebody who really needs it.
Look at the dog which is barking on that man!
He doesn’t want to read the book that I gave him.
I came to the city where I was born.
It’s common to use that instead of which, who, whom, especially in informal speaking.
We can omit the relative pronoun if it is the object of the verb.