Prepositions are connecting words that show the relationship between nouns or pronouns. They precede a noun or pronoun and indicate its relationship to another part of the sentence. Common examples include words like on, in, under, beside, behind. The document discusses different types of prepositions categorized by their function, including prepositions of place, time, direction, agent/instrument, purpose/source, and measurement/possession. Learners are advised that prepositions must be understood in context as there are no hard rules for their usage.