A preposition is a word which shows relationships among other words in the sentence. The relationships include direction, place, time, cause, manner and amount. The preposition is almost always before the noun or pronoun and that is why it is called a preposition.
Preposition of time
(before, around, at, during)
He was caught cheating during the exam.
Preposition of place
(in, on, at, to)
Umiey has lived in Australia for three years.
Preposition of position
(above, across, against, along)
The boys ran across the road even though there was heavy traffic.
Preposition of manner
(at, with, by)
He is good at learning foreign languages.