Definition
A preposition expresses a relationship between a noun (or pronoun) and some other part of the sentence.

It usually tells us where something is.

Examples:
with, out, in
under, over, around

Use:

A preposition is used with a noun or pronoun.
• The child ran around the snowman.
• Jack and Jill went up the hill.

However, the same words are adverbs in the following statements:
• Let's take a walk around.
• My lucky number came up.
• He came over to me.

They are adverbs because they tell us about the verb.
Prepositions are mainly used in English to form adverbial and adjectival phrases, as in the following:

Adverbial phrases

'Marseilles is in France'
['in France' tells us where Marseilles is]

'Hastings stands on the south coast of England'
['on the south coast of England' tells us where Hastings stands]

'The grocer marvelled at the arrival of the boxes'
['at the arrival' tells us when the grocer marvelled]

'She left the hall with a toss of her head'
['with a toss of her head' tells us the manner in which she left]

All the prepositions above are used adverbially to tell us more about the verb in each case.

The following are examples of adjectival phrases. In each case the preposition describes a noun:

Adjectival phrases

'The first cable across the Atlantic was laid in 1838'
['across the Atlantic' describes the cable]

'I love the sound of the sea'
['of the sea' describes the sound]

'I believe that the man in the moon exists'
['in the moon' describes the man]

'We all enjoyed the cheese on toast that our mother gave us'
['on toast' describes the cheese]

Prepositions are usually used, as in the two sets of examples above, with a noun or a pronoun.

Examples of nouns from the sentences given above are 'the arrival', 'a toss', ' the moon' and 'toast'.

Prepositions can also be used as adverbs without an accompanying noun or pronoun.

Come in, Turn round, Go up
Jump off, Look around, Go under