There is a very good discussion of the Ivy Academic Index at the New York Times.
For those not familiar with the general background of the Academic Index, it is an academic formula used by the Ivy League Colleges to determine which athletic recruits will be admitted. Recruited athletes need to meet a certain minimum academic score to be admitted to one of the Ivy League colleges.
While the Academic Index is used only for recruited athletes, it will give you some idea of the ways that selective colleges like the Ivy League will accept certain students who may not be as strong academically at they typically accept but who have other abilities of interest to the college.
In college admissions, your academic ability is but one issue considered by the colleges in deciding who to admit. While some people may disagree with this approach, it is the reality of US college admissions.