A preposition shows the position of an object. A preposition always begins a prepositional phrase. The phrase ends with an object of the preposition. An object is always a noun, so we use a noun for the object of a preposition.
Look at this sentence: Bob ran up the stairs.
The words up the stairs form a prepositional phrase.
The word up is a preposition. The word stairs is a noun which acts as the object of the preposition.
Look at more sentences with a prepositional phrase:
The car drove across the bridge. The noun bridge is the object of the preposition.
I see a spider on the ceiling. The noun ceiling is the object in this sentence.
The ball rolled under the chair. The noun chair is the object of the preposition here.
In each sentence, the noun at the end of the phrase is the object of the preposition.
The noun at the end of a prepositional phrase is called the object of the preposition.
▶Now you try it. Find the object of the preposition in this sentence. There is a fly in my soup. Did you find the noun at the end of the prepositional phrase? Good for you!