Impaired glucose tolerance simply means that blood sugar (glucose) levels don't return to a 'normal' range within a given amount of time. For instance, two hours after eating (post prandial) OR drinking carbohydrates a non-diabetic's blood sugar level would be expected to fall below 7.8 mmol/l [140 mg/dL]. If it doesn't, this would indicate that their body isn't able to efficiently deal with a sudden influx of glucose ... i.e. that their body is intolerant to glucose. (e would normally obtaing glucose from the breakdon of carbohydrates during the digestive process. Doctors, however, would give a measured amount of glucose, in a sickly sweet drink, that needs to be drunk when performing an OGTT (Oral Glucose Tolerane Test).
Many non-diabetics' blood sugar levels, at the two hours post prandial testing would have it return to their pre-meal level.
Some people can also have what's called an impaired fasting glucose (IFG) level. This is where the first blood glucose level check, when they first get up, is higher than the 'normal' range of 3.9 to 5.5 mmol/l [70 to 99 mg/dL]. This occurs because the liver releases some of it's stores of glycogen ... the way it stores glucose ... into the bloodstream in preparation for the day's work ahead, usually in the early hours of the morning. (This happens with everyone, but with someone that has a problem with glucose metabolism, the blood sugar (glucose) levels ould remain higher than the 'normal' range.)