Since I do not have an account, nor do I wish to register for one, I am not sure how it works, Is it comparing you against students who were accepted, or against those who were accepted versus those who were rejected (what percent of students with your profile were accepted versus rejected). If the latter, it can be a very rough idea as to where you are, stats-wise.
Unfortunately, for colleges like Brown with holistic admissions, stats are only part of the picture, and nothing else is really quantifiable.
All that the calculator will be able to tell you is what you will get here if you post your profile - Brown (for example) is a reach for anybody, and the higher your stats (in general), the more likely you are to be accepted, the more intense your ECs are, the more likely you are to be accepted, the more awards you receive, and the more prestigious they are, the more likely you are to be accepted, the more impressive your accomplishments - the more likely you are to be accepted.
However, since the number of applicants who have similar profiles to you also determines your chances, and nobody knows who else will be applying when you do. So that is another source of uncertainty.
Moreover, even though an applicant with a GPA of 3.95, and SAT of 1580, five great ECS, eight great awards, etc, is more likely to be accepted than an applicant with a 3.89, and SAT of 1480, fewer awards, fewer or less intense ECs, etc, some applicants with profiles like the former will get rejected, while some applicants with profiles like the latter will be accepted.
So, at very best, the calculator, and people here, can tell the proportion of people who had profiles like yours were accepted. Moreover, all that the calculator and the people here have is a rough approximation of your profile and those of the other people in the past who have applied.
Nobody will be able to tell you whether you will be accepted, since nobody is 90% accepted or 25% accepted - you are accepted or you are not accepted. Probabilities are continuous, outcomes are discrete and binary.