Prepositions are words that describe the relation between other words in a sentence. These are usually spatial and temporal relations. The word 'preposition' is formed by joining 'pre' and 'preposition'. They are generally placed before nouns or pronouns. For example:
The thief ran across the garden and jumped over the wall.
The words 'across' and 'over' tell relations between different words in the sentence and are placed before the nouns 'garden' and 'wall' respectively.
There are many kinds of prepositions. for example; prepositions of place, prepositions of time.