Incomplete question. However, I inferred you need help understanding when a word is categorized as a preposition.
<u>Explanation</u>:
In simple terms, a preposition refers to a word or group of words in a sentence that is placed before a noun, pronoun, or noun phrase in other to tell the time, direction/location, or place of an object.
Common examples of words to look o for that can act as a preposition in a sentence include: