A preposition is a word that helps to explain the context (where in space, when in time, what group it belongs to) of a noun. In the phrase, "I never understood the value of studying until I went to college," the word "until" helps to explain the timing of the author's realization; the word "of" helps to classify studying as one of the author's values, and the word "to" helps to explain location of the author.
So, three prepositions in that sentence are "of," "until" and "to."