Objects of prepositions are nouns that come after a preposition. For example,
EX: I went into the hotel.
EX: They asked me about Max.
EX: The car is parked behind the house.
Prepositions never stand alone. They need an object. The two go hand-in-hand. If you spot a preposition, you'll know its object is close by. (Here's a way to find prepositions quickly and easily. Ask, is it something a cat can do? For example, can a cat go "into", can a cat go "about", can a cat go "behind"? Why memorize a list? Use the "cat test". )
Coordinating conjunctions join words or phrases or sentences:
I like books
=> I read and I like books.
Subordinating conjunctions add information. The clauses they introduce cannot stand alone:
because I like books.
=> I read because I like books.
If you say, "because I like books" on its own, without additional context, your audience won't know what you mean. "because I like books" cannot stand alone. It needs more.