Body mass index (BMI) is an approximation of an average adult's body fat by calculating a ratio between your weight and your height.
While it is a good estimation for most people, it is rather inaccurate for people with low body fat or who are muscular. You should take this value as a suggestion and not as the absolute truth.
You can use this calculator to work out your Target Heart Rate Training Zones (You need to know you Maximum Heart Rate (HRmax) and your Resting Heart Rate (HRrest)
You can use this calculator to estimate your VO2 Max using the Pedersen Method (You need to know you Maximum Heart Rate (HRmax) and your Resting Heart Rate (HRrest)