A condition where a person's blood contains an abnormally high level of blood sugars (glucose). Glucose levels above 7 mmol/L going into a meal, and over 10 mmol/L two hours after eating, are generally considered too high, and must be brought down. Levels higher than this may be absolutely without symptoms; it's possible to feel perfectly well, but have glucose levels that are too high. Over periods of months and years, such high levels will predispose an individual to all of the complications of diabetes.
There are many exceptions to this, so you should ask your doctor what your glucose levels should be.