Prepositional phrases describe the relationship between and object and the preposition used with it.  Words such as inside, on top, around, all, any, more are examples of prepositions and can be used in prepositional phrases.  If these words do not have an object when in a sentence, then such words are not functioning as prepositions.
Prepositions are used with a noun or a pronoun to show the relation of this word to another. A preposition with an object, which is usually either a noun or a pronoun, is called a prepositional phrase.
These are some of the words which are prepositions when used in this way: above, across, at, below, beneath, between, by, regarding, since, toward.

examples of prepositional phrases

above the clouds
across the Rio Grande River
at the dentist
below the radar
beneath the sky
between the sheets
by the mountain
regarding your exam results
since 2005
toward the end

There is a bit more to prepositional phrases, however.  The above examples show nouns with prepositions.  A prepositional phrase also occurs when the noun is not an actual noun, but in the context of the sentence, it acts like a noun.t is a little bit more complicated than shown above because the noun can be anything that plays the role of a noun. For example:
near him (him is a pronoun)
since marrying (the ‘noun’ is a gerund)
around 5pm that afternoon (here is a noun phrase)
for winning the race (noun clause)
These are prepositional phrases because they obey the basic rule of prepositional phrases in that the words following the preposition are the object of the preposition.

Modifiers can be used as well in prepositional phrases, such as
around 5 pm that stormy afternoon

Prepositional phrases can also function as adjectives or adverbs.
I reached the mountain peak although exhausted.

Sentences are full of prepositional phrases. Prepositions, when combined with an object, which, as well as nouns and pronouns can also be combined with adjectives or adverbs, gerunds, noun clauses and phrases, are called prepositional phrases.