In languages all across the world people need to describe things. English writers often need to modify nouns or pronouns so they use words like: below or inside. Using a prepositional phrase is simple, just try to describe what you are talking about and "boom" you did it. A good example would be: The dog ran inside the house.
For more information on Prepositional Phrases visit: http://grammar.yourdictionary.com/parts-of-speech/prepositions/Prepositional-Phrases.html