All drivers who have had a drunk driving conviction should have their licenses revoked.
All drivers should have their licenses revoked.
The highlighted clause is essential because the meaning of the sentence is changed drastically if the clause is removed from the sentence. A restrictive clause is not set off with commas.
However, a nonessential or nonrestrictive clause needs to be set off by commas. A clause is nonrestrictive if it simply adds information that is not essential to the basic meaning of the sentence. If a nonrestrictive clause is removed, the basic meaning of the sentence is not changed.
My father, who is still farming, is 74 years old.
My father is 74 years old.
The highlighted clause is nonrestrictive. If it is removed from the sentence, the basic meaning of the sentence is not changed. Nonrestrictive clauses usually begin with one of these subordinating conjunctions: who, whom, whose, which, or that. (Technically, the proper subordinating conjunction for a restrictive clause is that, while nonrestrictive clauses use which, but in practice many writers ignore this distinction.)