Metabolic syndrome is the name we now give to the combination of obesity and pre-diabetes that is seen in many people, especially those with visceral fat (fat around the organs).
Why does this matter? Because it is strongly associated with cardiovascular risk, with chronic inflammation throughout the body, with risk for overt type-2 diabetes and a lot of other very grave and progressive health problems.
What's worse is that this syndrome can develop in people who appear to be thin but have the "TOFI" body type. TOFI stands for "Thin on the Outside----Fat on the Inside" These are people whose BMI is normal, who have very little visible subcutaneous fat but who have, hiding inside the abdominal cavity, large amounts of visceral fat. TOFI, by the way, is one of the many examples of why BMI is such a terribly flawed measure of health.
At any rate, for those of you who are interested, Nature Medicine has generously posted a superb monograph explaining metabolic syndrome in detail. I strongly recommend it.