What is a preposition?
A preposition is a part of speech, or word, that modifies other parts of speech and is part of a phrase that ends with a noun called the object of the preposition. Prepositions must always be followed by the object of the preposition.
A phrase that begins with a preposition and ends with the object of the preposition is called a prepositional phrase.
Because there are approximately 150 prepositions in the English language, no one is expected to memorize every preposition. Instead, we rely on the sentence structure itself to help us identify prepositions and their objects. (If you want to see a list of prepositions, go to the 46-second mark in the video link provided.)
Example: I walked into the store.
The prepositional phrase "into the store" modifies the verb "walked," answering the implied question of "Where did I walk?" This person walked "into the store." The word "into" is the preposition, and the word "store" is the object of the preposition. This phrase "into the store" is, therefore, the prepositional phrase.
Example: They made their announcement in the room with the most seats.
The above example actually has two prepositional phrases.
The first is "in the room," which modifies the verb "made" by telling us where the announcement was made.
The second is "with the most seats," which modifies the noun "room" by telling us which room.
If you need further assistance with understanding prepositions, watch the video provided in the link.