A preposition is a word which is placed before a noun or a pronoun to show in what relation the person or thing denoted by it stands to something else in the sentence.
When a preposition is used after the object:
(i). Here is the book that you were looking for.('that' is the relative pronoun object for preposition 'for').
(ii). What are you worried about?('what' is the interrogative pronoun object for preposition 'for').
When the preposition is used before the object:
(i). This I insists on.
(ii). He is known all the world over.