This is a sentence fragment. A full completed sentence with this wording would be "If your parents think today's fashions are weird they should see the clothes people wore in the middle ages."
This is not a complete sentence, when a sentence is fragmented it has disconnected from the main clause. The best way to fix a fragment is to remove the period from the two sentences if they are together.
Other types of punctuation are allowed for sentence fragments such as commas.
