As the linked handout explains, "A word used to show the relationship of a noun or a pronoun to another word in the sentence is a preposition." To explain, prepositions are words that express how nouns and pronouns connect to other ideas -- usually a noun or verb -- in a sentence. Usually, this connection relates to time and space.
Regarding prepositional phrases, the linked handout states that they are "A group of words beginning with a preposition and ending with a noun or a pronoun." Almost every time you see a preposition, it will be part of a prepositional phrase, which means it is attached to a noun or pronoun. Below are some examples with the prepositional phases italicized. Note how they all start with a preposition, end with a noun or pronoun, and focus on describing a noun or verb, usually by referring to time and space:
Before dinner, the cat was sleeping behind the sofa.
I cannot stand our neighbor across the street and down the road.
One of my best friends knows everything about comic books and works in the local comics shop.
For more specific information on and examples of both of these concepts, please see the linked handout: