A relative clause fragment arises when this type of clause is disconnected from the other clause that is needed in the sentence.
To make sense, a sentence must express a complete thought. A relative clause written by itself does not do this.
These are all dependent clauses.
A clause is a group of words with a subject and verb. Like any complex sentence, a sentence with a relative clause is made up of at least two clauses, a dependent clause and an independent clause.
This is a definition of 'dependent' from the Cambridge Online Dictionary:
An independent clause can be used alone. However, a dependent clause needs the support of an independent clause in order to exist, or to make sense.
So now let's add some independent clauses to those relative clause fragments we looked at earlier on to ensure they express a complete thought.
As every clause has a subject and verb and you are using two clauses (an independent and dependent clause) check that you have two subject verb combinations.
My neighbour (subject) often helps (verb) me out
who (subject) is (verb) a policeman
I (subject) went (verb) to the chemist
where I (subject) bought (verb) the medicine my sister needed
Both the independent and dependent clause have a subject and verb so the sentences look correct.
Note that the relative pronoun (who, which, that etc) is not always the subject of the sentence, as in the second example, but that will be dealt with further in another lesson.