Prep. is only a short way of writing preposition on the page. You see it in dictionaries, grammar notes, and word lists, almost never in a normal sentence. If a line says in prep., it is telling you the job of the word in grammar: in, on, at, of, with are prepositions — little words that sit before a noun and show place, time, or relation. Teachers and books use it to save space. Though actually, you would not read it out as “prep” in class as if it were a word of its own — you would say “preposition.” It is a writing mark, like n. for noun or adj. for adjective. Spoken prep is a different shortening, of preparation: exam prep, prep work, prep school. That is not this entry. This prep. with the full stop is the grammar label.