BMI, or Body Mass Index, is a formula that is supposed to provide a better overall health indicator than weight alone. In other words, people of different sizes should way different amounts. The BMI formula addresses this.
Further, there are charts — based on population averages — that report on whether or not a person is overweight or, perhaps even, obese.
The WildFit stance on the BMI system is that it is, like weight in general terms, a useful indicator but not an accurate diagnostic took. People with high muscle mass, for instance, might indicate as overweight or obese even when they are not.