A preposition is a word that establishes a relationship between a noun or pronoun and other elements in a sentence. It often indicates direction, location, time, or manner, such as "in," "on," "at," "by," and "with." Prepositions are typically followed by a noun phrase, forming a prepositional phrase that adds detail to the sentence. For example, in the phrase "on the table," "on" is the preposition linking the action to the location.