In the sentence "the newscaster filed her reports about the riots," "about" is the preposition.
A preposition is a word employed in order to link nouns, pronouns, or phrases to other words within a sentence. They connect the people, objects, time and locations of a sentence.
Prepositions are words which are heads of prepositional phrases. A prepositional phrase refers to a group of words which include a preposition, a noun or pronoun object of the preposition, and any modifiers of the object.
About, as a prepositions, means on the subject of; concerning.
