Question

What is preposition?

Answer 1

A preposition is a word such as after, in, to, on and with. prepositions are usually used in front of the nouns or pronouns.

Answer 2

Preposition comes before a word which it complements. 
Prepositions usually express relations (in terms of space and time), of the word which is following it.
Words like "from", "for", "until", "in" are examples of prepositions.