Prepositions are words that show the relationship between a noun or pronoun and other words in a sentence. They indicate location, time, direction, and other relationships. Common prepositions include in, on, at, by, for, from, to, with. For example, "The book is on the table" - "on" shows the relationship between the book and the table. It's important to use prepositions correctly to convey accurate information about the relationship between things in a sentence.