Prepositions are linking words. Prepositions are very important words - they link words together in a way that makes sense. Prepositions come before a noun (or pronoun) to explain its relationship to other words in the sentence. In the sentence, 'I gave the medal to the best runner', the preposition 'to' explains the relationship between 'the runner' and 'the medal'. Prepositions often describe time (i.e. 'after' and 'before') and place (i.e. 'behind' and 'below').
Try this quiz on prepositions to check your understanding.