The preposition is an invariable part of the sentence that establishes a relation between two words. They don't have a meaning of their own and show position, time, relationship, etc. between words.
Examples of the construction
preposition + noun are shown below:

*Note: Most of the time the noun is modified by an article or an adjective.

Yo prefiero el bocadillo sin queso.
I prefer the sandwich without cheese.

Tenía los codos sobre la mesa.
He had his elbows on the table.

Él estaba cantando bajo la lluvia.
He was singing under the rain.

Suba por la escalera.
Climb the ladder.

Reservaremos literas hasta Madrid.
We will reserve bunks to Madrid.

El bolso está tras el puesto.
The bag is behind the stall.