Prepositions are words that show the relationship between a noun or pronoun and other words in a sentence. They often indicate location (on, in, at), direction (to, from), time (before, after, during), and manner (with, without). Prepositions help provide context and clarify the relationship between different elements in a sentence. For example, in the sentence "The book is on the table," "on" is the preposition showing the relationship between the book and the table. Understanding prepositions is vital for mastering English grammar and constructing coherent sentences.