A preposition is a word that is used before a noun or a pronoun to connect it to another word in the sentence. It is usually used to show location, direction, time, and so forth.
Examples: On, in, at, by, under, above, beside, to, out, from, for.
I sat on the floor.
Let’s go into the house.
We will meet at four o’clock.
Have a look under the couch.
He went to school.
This letter is for you.
The word “preposition” comes from the Latin word praeponere (put before). So prepositions usually come before the noun/pronoun.