The Fat-Free Mass Index is a measure of how much muscle mass a person has per unit of height. It is sometimes used to help determine if someone is on anabolic steroids although this method is not 100% accurate. People with a FFMI of 25+ are more likely to be on steroids, and people less than 25 are more likely to be natural.
Some people believe that the FFMI is a better indication of health than the ever more popular Body Mass Index (BMI) because it takes in to account body fat percentage rather than actual weight. However, in my opinion, if you know your body fat percentage already then the FFMI is meaningless.
So what does 'athletic' mean since I'm overweight?
Absolutely nothing. These are terms that the creators of that website have decided to attribute to specific values of the FFMI. It's not actually a standard that people follow. They're meaningless except for within the confines of whatever message that website is trying to promote.
Does my body have the growth potential to have an athletic body?
I believe your body has the growth potential to have "an athletic body", but not based on the FFMI. Most people are capable of being more athletic than they currently are.