Body Mass Index is a simple statistical measure involving a person's height and weight. It is best used as a general guide.
Individuals with a healthy BMI are between 18.5 and 25 - that's usually considered optimal weight. Those below 18.5 are likely underweight, while those under 17.5 may have an eating disorder. People with a body mass index over 25 are often overweight, while those with a BMI over 40 may be morbidly obese.
There are special cases such as some bodybuilders where BMI is less useful.