It is a single word or phrase that must come after a noun. It modifies the noun in some way, like position, logic, etc.
There are over 100 prepositions, but the most common are
Here is an example.
The car went over the hill.
So, hill is noun that comes after over, which is one of those special words on the list. That makes it a preposition. "over" gives location information about the hill, which is a common thing prepositions do. The entire phrase "over the hill" is called a prepositional phrase and they are never complete sentences. If it does make a complete sentence, you can rule out the word being a preposition.
Steve laughed at the monkey.
"at" is the preposition and monkey is the noun. Hope this is helpful.