We use relative clauses to give additional information about something without starting another sentence. By combining sentences with a relative clause, your text becomes more fluent and you can avoid repeating certain words.
Defining relative clauses are composed of a relative pronoun (sometimes omitted), a verb, and optional other elements such as the subject or object of the verb. Commas are not used to separate defining relative clauses from the rest of the sentence. Commas or parentheses are used to separate non-defining relative clauses from the rest of the sentence.