People are considered obese when their Body Mass Index is not in proportion to their weight and height
I hear the BMI index isn't very accurate for athletes. Is this true?
yes, that can be true.  Some peoples metabolisms are different depending on how much energy they are using.
What is considered a healthy BMI to have?
A Healthy BMI is always in accordance to dividing persons weight by their height, if its over the range, they are overweight.