A relative clause is a kind of dependent clause that adds information about a noun in the main clause. Because relative clauses help describe nouns, they are sometimes called adjective clauses.
The underlined clauses in the sentences below are all relative clauses.
1. Marta is the manager of the new restaurant that serves tapas.
2. The restaurant that serves Latin American food is looking for a new waiter.
3. They serve grilled lamb, which John loves, at the Middle Eastern restaurant.
4. Manny is the manager of the Middle Eastern restaurant, which is open every day.
The relative clauses in Sentences 1 and 2 are called restrictive clauses, because they identify (or restrict) which specific noun is being referred to. For example, in Sentence 1, the relative clause tells which restaurant Marta manages: It is the one where they serve tapas. In this case, we can assume there is only one restaurant in the area that serves tapas, and that is the one where Marta works.
The relative clauses in Sentences 3 and 4 are called nonrestrictive, because the information they give does not identify (or restrict) which specific noun is being referred to in this sentence. For example, in Sentence 3, the relative clause adds the information that John loves the lamb, but this information isn't necessary to identify which lamb is being referred to. We already know that it's the grilled lamb.
There are lots of other things to learn about relative clauses, but this is a good start. I hope it helps.
