A preposition is the part of speech that shows a relationship between words. Think of “anywhere a cat can go.” A cat can go “under” the table, “on” the bed, or “out” the door. Looking for a cat? Use a preposition.
In the sentence "The car went careening into the ditch," the preposition is into: it's the word that points to the noun. "We love to chase kids around the carnival in our scary clown costumes" has two prepositions, around and in. They tell you where things are happening. The most common prepositions are of, to, in, for, on, with, as, by, at, and from.