People with diabetes have trouble with a hormone called insulin. Insulin, which is made in the pancreas, lowers the level of glucose (a type of sugar) in the blood. It does this by helping glucose enter the body's cells. Glucose is the main source of energy for the cells. And because you're made up of cells, you want those cells to get the fuel they need!
Your body gets glucose from the food you eat, and it travels through the bloodstream. But without insulin, glucose can't get into the cells. In diabetes, the pancreas doesn't make enough insulin or the body can't respond normally to the insulin that is made. This causes the glucose level in the blood to rise.