Selective schools use waitlists so they can fill up their classes even if not enough accepted students enroll in the freshman class. So you definitely can get in from a waitlist, but it's hard to gauge how likely it is because it depends on how many accepted students enroll.

Not all schools use wait lists, but about 10% of applicants (on average) are waitlisted at schools that do. So at least you sort of know where you stand. You're in the next 10% of people that didn't get in, but you could be anywhere in that 10%. Wait lists are really hard to gage because of this, so you shouldn't count on getting in.

My brother got waitlisted by Washington University in St. Louis, and no one in the family had ever heard of that before. We just called them and they explained to us what it meant and what the chances of getting in were. Eventually, he got into his first choice (The University of Chicago), so we just responded that he didn't want to be on the waitlist. So I'm not sure if he would have gotten in or not, but probably not. The admissions office told us that the chances weren't great.