There are two types of relative clauses using relative pronouns.
The President of the United States, who is visiting Moscow, claimed that relations between the two countries were at their best for twenty years.
NB - you cannot use that here (after a comma).
The intermission, which lasts for fifteen minutes, comes halfway through the film.
In this type of relative clause, the information is not essential; it could be deleted without making the sentence ungrammatical and it would still be clear who or what we are talking about.
The man that stole my car was fined. (this is used in American and British English)
The man who stole my car was fined. (this is used in British English)
The company which made it has gone bankrupt.
The company that made it has gone bankrupt.
In this type of relative clause, the information is essential; if it is deleted,then the sentence will no longer make sense as we will not understand who or what is being talked about.