A preposition is a word that expresses the relationship between a noun or pronoun and another word in the sentence.
A preposition does not function by itself. A preposition must have an object of the preposition, which is the noun or pronoun that relates to something else in the sentence.
A prepositional phrase is made up of the preposition and its object, along with any modifiers of the object.
Examples of prepositions:
Examples of prepositional phrases (the preposition is underlined and the object of the preposition is italicized):
at the beach
in the blue chair
across the highway
during the school's play
before the football game
outside the front gate