A preposition is a word that establishes a relationship between a noun or pronoun and other elements in a sentence, often indicating direction, location, time, or manner. Common examples include "in," "on," "at," "between," and "with." Prepositions typically precede their objects, forming phrases that provide additional context to the sentence. For instance, in the phrase "on the table," "on" is the preposition linking the object "table" to the action or context.