Prepositions are words that show the relationship between a noun or pronoun and other words in a sentence. They often indicate direction, location, time, or manner. Common examples include in, on, at, under, and between. For instance, in the sentence "The cat is under the table," the preposition "under" describes the position of the cat in relation to the table. Prepositions can be simple or compound. Simple prepositions consist of one word, while compound prepositions are formed by combining two or more words, such as in front of or next to. Understanding prepositions is essential for constructing clear and accurate sentences in the English language.