There is a whole lot of information about NAPS here.. Bottom line, successful completion of NAPS is "almost" assured acceptance to USNA next year. (The "almost" is only because there are exceptions to every rule....in the rare case of non-acceptance, there is usually a real good reason; and most of the time , the persons who would have a problem self-select by dropping out of NAPs).
I know that for an 18 year old, an offer to NAPS instead of direct appointment may seem like a rejection, or a wasted year. There is nothing further from the truth. I know -- I've been there, and I can look back almost 40 years and unequivocally say it was the best thing that ever happened to me. For details ...see my earlier comments on the subject.
PS .. CAPT MJ posted while I was typing; unless things have changed ..NAPSters enlist and start at E-1 (with promotion to E-2 around Christmas). That's pretty good money for a kid right of high school, with no real expenses to deal with.