The term "diabetes mellitus" refers to a group of diseases that affect how your body uses blood glucose, commonly called blood sugar. Glucose is vital to your health because it's an important source of energy for the cells that make up your muscles and tissues. It's your brain's main source of fuel.
If you have diabetes, no matter what type, it means you have too much glucose in your blood, although the reasons may differ. Too much glucose can lead to serious health problems. (Mayo Clinic)