A preposition links a noun or pronoun to another word, showing a relationship like place, time, direction, or manner: The keys are on the table. Here, on tells us where the keys are.
Picture someone shouting across a crowded kitchen: Put the milk back in the fridge! The word in links the milk to the fridge, telling you exactly where the milk should go. The whole group, in the fridge, is called a prepositional phrase: the preposition, its object, and any words describing that object.
Now imagine a text from a friend: I will meet you near the station at six. Focus on at six here. The word at takes the noun six and links it to the rest of the sentence, pinning down when the meeting happens. Each little word anchors a noun to the rest of the sentence, whether that anchor is place or time.
Prepositions and conjunctions are not the same job
A preposition points to a thing. I will call you after lunch. Here after links the calling to lunch, one noun.
A conjunction links whole clauses, each with its own subject and verb. I will call you after I finish work. Now after joins two complete statements.
The easiest test is what follows the little word. If a noun or pronoun comes next, it is working as a preposition. If a full clause comes next, it is acting as a conjunction. One wrinkle: when nothing follows the word, it can still be a preposition, because its object may appear earlier in the sentence, as in Think of it before you answer compared with a case where the object is simply implied by the situation.
Words like before, after, and since can do either job, so the rest of the sentence tells you which one you are looking at.
Try it on this pair. Compare We talked before dinner with We talked before we ate. In the first, before points to the noun dinner. In the second, before links the whole clause we ate. Same word, two different jobs.