Many people who are not diabetic but who will develop type 2 diabetes, do so because of what they eat and because of a lack of activity. Diabetes itself is diagnosed purely on the basis of blood sugar levels, though the disease is much more complicated than that. Prediabetics have a clear signal, a warning, that the insulin that they are producing is not coping adequately with the type of carbohydrates that they are eating. You won't know why. It could be one of the genetic types of diabetes, or a failing pancreas due to a separate illness but mostly, it is due to some insulin producing islets getting blocked, a slow insulin response to raised blood suagr levels, an increase in insulin resistance or a combination of these factors. Increasing your activity will help with muscle insulin resistance, watching what you eat, avoiding all highly processed or refined carbohydrates will control what glucose is added to your blood and losing weight will help somewhat to unblocking fat deposits in your liver and pancreas.
Take it as a warning and don't wait for it to get worse. There are many on this forum who would have loved to get a warning but who jumped from knowing that they might get diabetes to actually being diagnosed with it. They nearly all feel like kicking themselves. You can most likely avoid that.