I am going to quote a lot from an old-school book on English grammar that explains prepositions, among other thing, in a way the is clear and easy to understand.
Certain words in a sentence relate nouns and pronouns to other nouns or pronouns, to verbs, or to modifiers. The words are called prepositions.
The rule is: A preposition is a word that shows the relationship of a noun or pronoun to some other word in the sentence.
Look:
The St. Bernard slept NEAR my bed. The St. Bernard slept UNDER my bed. The St. Bernard slept IN my bed.
The prepositions in the above are: near, under, and in. The vary in meaning as they relate to the St. Bernard. See how they "modify" the dog? They answers questions like an adjective or verb would: What kind? Which one? How much? How many? Where? When? How? etc.
Let's look at one preposition from the dog sentences: The St. Bernard slept in my bed. The noun "bed" is being related to "St. Bernard" and the preposition "in" is explaining how or what way it is being related by answering the question, Where? but not only where, exactly where the dog slept.
The preposition and the noun or pronoun that follows combine to form a prepositional phrase. in = proposition the =article modifying bed bed= the noun that follows the preposition
Now the hard part. There are two kinds of prepositional phrases: an adjective prepositional phrase and an adverb prepositional phrase. Each answers a certain kind of question.
The adjective prepositional phrase answers questions like an adjective: What kind? Which one? How many? etc.
and the adverb prepositional phrase: Where, when, how?
Keep in mind an important rule: a word, phrase, and clause all act as ONE word. The idea of a phrase acting as one word is in your mind with the questions being answered:
The St. Bernard slept in my bed. Where did it sleep? in bed. bed. bed = the noun being related to the dog, if you recall the rule "the relationship of a noun or pronoun to some word in the sentence." bed related to St. Bernard
What gets hard is that sentences are filled with these prepositional phrases.
He bought the best the most expensive car in the showroom at the back of the mall on Tuesday.
Let me and anyone else, get a grammar book and learn more about prepositional phrases and how they work. There's more to learn about them here mentioned by others, and there's place online that will give you more info and practice. Use Google or Bing or some other browser and type: prepositional phrases. You discover many grammar sites when more on the subject as well as practice exercises. You know what a library is? Go there. Use the Dewey Decimal number 400-420 area to find books on English and grammar. Good Luck.
Here's a few more sites to help you:
http://grammar.ccc.commnet.edu/grammar/index.htm
http://www.chompchomp.com/terms/prepositionalphrase.htm
This one is fun and easy with quizzes: