A preposition is a relating word placed before a noun or pronoun to show its relation to some other word in the sentence.
For examples: In, On, At, From, To, By, Outside, Under, Below, Along, Around, etc.
He is born in Mumbai. He is running on the field. She stays at home now. He started his journey from Chennai. He ended his journey to Mumbai. She went home by car. He is talking on the mobile outside of his office. Her purse is under the table. The temperature is below average today. She walked along the highway. She walked around the park.