The cat sat _on_ the mat.
* _on_ is the preposition.
* It shows the relationship between the cat and the mat (where the cat is located).
Prepositions are words that show the relationship between a noun or pronoun and other words in a sentence. They usually describe location, direction, time, or manner.
Here are some other common prepositions:
* at
* in
* under
* around
* through
* before
* during
* after