Question

What is a prepositions ?

Answer 1

PrepositionsA preposition is a word which precedes a noun to show the noun's relationship to another word in the sentence. (The word preposition comes from the idea of being positioned before. It is not true to say that a preposition always precedes a noun or a pronoun, but it does most of the time.)

some ex.
above, about, across, against, along, among, around, at, before, behind, below, beneath, beside, between

Answer 2

preposition is a part of parts of speech .    in gramar preposition  is a type of adposition  a grammar partical  that establishes  a relationship between an object ( usually a noun phrase ) & some other part of a  sentence , often expresing a location or time . 
examples 
1. my cat on  is the sofa
2.i knitted the throughout day
3.they will be not finshed   until lunch time
4.the keys are between the cusions
5. a man hid behindthe door